As of today, 2/15/2012...I've improved the item build, given additional details and more guidance. I hope you like the changes! Please comment when you get the chance and remember, do not vote until you try it out a number of times :)

The Greater Quintessences of Vampirism are not essential for Nocturn. He can jungle just fine with Quints of Desolation. However, with 3 Quints, your life stealing mastery, and a Doran's Blade you'll have 12% life steal. You can easily gank several times between levels 1-6. I highly recommend that experienced players follow the item build I've laid out, get a leash at red, and then gank bot or mid at level 2. Unless...you wanna sit in the brush bot with your two team mates for first blood and a possible double kill at level 1 ;). Continue ganking a lot...you should not wait until level 6 to gank if you follow the item build I've laid out.

If your just learning how to jungle with Nocturn then I suggest starting with a Vampiric Scepter or Cloth Armor with 3 pots. I'd still recommend starting at red. If you choose not to gank, then go for the wraiths, wolves and then blue. After that, feel free to roam back to wraiths, gank or heal up. If you go the cloth armor route...I suggest jungling until you hit 700 gold so you can buy Madred's Razors. After that, follow the item build I've laid out. Be sure to skip the Doran's Blade and eventually upgrade that Madred's Razor to Wriggle's Lantern or the Vampiric Scepter to Wriggles or a Blood Thirster. ;)

Although I haven't added them in at any specific point, do not forget to buy wards. Be sure to place them at their buffs if they have a jungler so you can gank them at will.

Once you get to the BF Sword (not the BF Sword from Infinity Edge) you should have 6 items in your inventory. At this point, you should sell your Doran's Blade to pick the BF Sword. Then, proceed to follow the item build.

Let's tackle some of the questions you may have about WHY I choose the items I choose. Let's start with your obvious question..."WHY ARE YOU BUYING 2 Infinity Edge's!?! Zomg, I'm going to have a heart attack over it!" First, calm down and let me explain. It's simple, it gives you 80 dmg and 25% crit...with two Infinity Edges you have ALL THE DAMAGE YOU'LL EVER NEED. Combined with Phantom Dancer and Executioner's Calling your crit will be at 95% with your base damage at about 287 and your life steal at 27%. Each shot you fire will be a crit and will do 746dmg+ without any buffs. If you get the red or baron buff...you can expect that to come close to the 1k mark. So, back to the question...why do we get 2 Infinity Edge's? Simple, we want max damage via having MAX CRIT! Who cares if you can get a higher base damage if you don't crit on every shot? Constant crits will always out damage any other option in my humble opinion. The only thing that somewhat affects your damage is armor. But honestly, armor doesn't matter much to Nocturn. The tank and possibly the bruiser are the only ones that get it and you should NEVER FOCUS THE TANK anyway. All you have to do is allow your tank to initiate while you remain mia and hover around the area. If the tank lands a great ult, pop your ult and zoom in on the primary squishy or support target and proceed to go through the team fight. The carries and support should die quickly. By that time your whole team will focus on the tank and/or bruiser and as you will observe...armor won't matter. Second scenario would be you hover around the area and notice one or two opposing team members out of place. You, instead of the tank, initiate with your ult and zoom in on the target while you and your team make quick work of them. Now you have a 5v4 or 5v3 team fight situation and your team should easily win that just by sheer numbers. Keep in mind that you should only initiate and zoom in on an opposing team member if they are NOT THE TANK. ZOOM IN ON THE TANK AND YOU MAY COST YOUR TEAM THE FIGHT!

Some of you may wonder why we buy Executioner's Calling instead of a Bloodthirster for the lifesteal. I buy Executioner's Calling because it gives you the last crit% you need to make every shot a crit and allows you to Grievous Wound your target every time you gank on top of a great % of lifesteal. You get all of that for the low price of 1350. It's a flat out BARGAIN! I'd say it's the most overlooked item in the game and easily one of the most powerful to use all game long because of Grievous Wound's scaling against life steal, hp regen and spell vamp. Almost every champ has some form of regen, spell vamp or life steal and this item reduces it by 50%! Honestly, I think this item is way too overpowered and way too cheap...hopefully it doesn't get nerfed like the Sword of Divine did ;)

This leaves us with the all important question, what's the last item I should get...IF YOU GET THAT FAR IN YOUR GAME!

Ah yes, how do we decide which item will be our last....

First of all, if you're jungling correctly with Nocturn then your games are ending well before you can buy your last item. If you have to buy your last item then one of two things is most likely true:
a) Your team had a very rough game. You had a feeder, there's a lack of coordination, or both.
b) You somehow didn't get fed because you failed your ganks, you failed to gank enough, or your team couldn't coordinate ganks well with you.

Either way, you're in the position of buying your last item. Observe the opposing team's items carefully and choose wisely. Defensive items are useful on Nocturn. He's a bruiser with high sustain, thanks to Umbra Blades and Shroud of Darkness (Your on-call Banshee Veil). Some people like to pickup Warmogs and others play him like a carry by doubling up on Phantom Dancers and picking up The Black Cleaver. Personally, I buy the Warmog's and the Phantom Dancer. Why? The Phantom Dancer completes the crit build and the Warmog's gives him excellent sustain. So much that he can off-tank quite well.

I follow the sequence I've laid out because it maximizes the lvl 1-6 ganks. Duskbringer is your bread and butter and must be focused early on.

Your ideal gank sequence starts at level 3. But you could get a lvl 2 gank opportunity. Get a leash at red and observe the bot and mid lanes. If you sense a gank opportunity, pickup your fear skill (Unspeakable Horror) at level 2. Then proceed to gank. Otherwise pickup Shroud of darkness and jungle until level 3 where you can start ganking at will. Your goal is to land Duskbringer on your target, followed by fear and then you attack away. If you sense a spell cast, pop your banshee veil (Shroud of Darkness) and proceed with the attack.

What you should keep in mind while ganking bot in the 3v2 situation is who to fear. If there's a tank or a support with a carry, then you'll want to land your skill shot on the carry and rush after them. While rushing them, fear the tank/support and keep your focus on the carry. Weather you get the kill on the carry or not the tank/support will then die since it's just become a 3v1 situation. Keep in mind the reason you fear the tank/support is to prevent their cc which could possibly allow both your targets to escape.

Your involvement with Team Fights is important to manage. Sometimes you're present, other times you let your 4 team mates push a lane. Meanwhile, you draw the opposing team to the lane you're back dooring because the other team hates you so much. Split pushing and back dooring as I've described are important to weave in to a game. However, be very careful about doing it if:
a) Your team had a very rough game. You had a feeder, there's a lack of coordination, or both.
b) You somehow didn't get fed because you failed your ganks, you failed to gank enough, or your team couldn't coordinate ganks well with you.

When you find the need to join team fights you must not forget the POWER OF YOUR ULTIMATE! Either you or your tank will be the one to initiate and your ultimate can easily cause your team to catch 1-2 people out of position and kill them before the other team can even respond. If your ult is up, it's best NOT TO BE INSIDE THE CLUSTER OF YOUR TEAM. Instead, remain mia and roam within ult distance of them. If you happen to find an opposing team mate out of position, pop that ult, launch yourself at the unlucky champ and enjoy the free kill and the new 5v4 team fight situation. There is one caveat. DO NOT ULT AN OUT OF POSITION TANK EXCEPT IN VERY VERY VERY VERY RARE CIRCUMSTANCES! You ult their tank and you can expect a quick 4v5 situation in favor of the other team. I DON'T CARE HOW MUCH YOU HATE RAMMUS OR WHATEVER TANK YOU'RE UP AGAINST. Don't give in to ulting him unless it's a very rare circumstance where you're absolutely sure he's alone, and you know you and your team will kill him before their team counter-ganks ;).

Whether you earned the new 5v4 team fight situation or not, your focus is to take down the enemy carry and/or support. This should be common knowledge but in case it's not...ignore the tank until the end of the fight.

This guide isn't completely exhaustive but is meant to give you a good idea on how to jungle with Nocturn. If you're looking for a fully comprehensive guide I'd suggest checking this one out: http://www.mobafire.com/league-of-legends/build/haunting-the-jungle-nightmare-style-109358

Although I disagree entirely with his main item build, he has a good point of view to consider on how to jungle with Nocturn.