I updated this on May 8, 2012 for the new mastery system. It doesn't work quite as well, but it still does the trick.

Welcome to this fun build for Pantheon. High health, armor, and some magic resistance combined with his passive makes him a great tank. Follow this guild to make interesting games with TANKTHEON.

Notice how I balanced the runes so they not only benefit tanks, but DPS/Crits as well? You need Attack Speed to reactivate your passive as fast as you possibly can. Combined with attack speed masteries, you will attack fast enough that you do not need attack speed items.

As weird as this might seem, Offensive Masteries are really good for this build. You have to at least get the Sorcery masteries for this build to function the way it needs to.

The second one demonstrates an alternative mastery page you can use. It is more utility based for regeneration of both mana and health.

Mana Manipulator: Seems weird that you should get mana regen as first item, right? Think about it; Spear Shot with a <4 cooldown = lot of mana drained really fast. Besides, it builds into Soul Shroud, which benefits all of your teammates around you (especially the casters).
Mercury's Treads: Magic resistance + Tenacity is what you need. Forget Berserker's Greaves, remember this is a tank build not DPS.
Frozen Mallet: This is great for every DPS and tanky champion. Keeps the enemies close, and gives you health. If you don't want to get Frozen Mallet, it is interchangeable with Warmog's.
Force of Nature: Life regeneration is great, especially as this guide ends with 3000+ HP. Also it gives you magic resistance, which is a necessity against any caster.
Thornmail: This is great if you are against DPS champions. You can swap the timing of FoN with Thornmail, depending on how you are holding up.
Atma's Impaler: Atma's should be your last item. This will add a little over 60 damage to your attacks, as well as crit chance.

Always get Spear Shot first. You will need it for harassing, minion and champion killing. Only get one level in Heartseeker Strike until you have maxed all of your other skills. This is because you won't have enough damage to make its active to make it deal a lot, but its passive will help you demolish low health enemies.

My Preference:
Ghost: I use Ghost as a summoner spell no matter who I play. The speed bonus is a good chasing down tool as well as running away.
Exhaust Exhaust is useful to keep enemies in the fight longer or to keep them from dealing damage to you or an ally.

What you can get:

Flash: Flash is an ok summoner spell to get with Pantheon. It can either put you in range for Aegis of Zeonia, or it can help you escape.
Ignite: Good for ensuring a kill, but that is about it. Where you aren't going to be dealing a lot of damage, it is up to you to decide if it is worth it or not.
Heal: It is good at the first, but in late game it won't do enough to make a large difference.
Cleanse: Only good if you are against a lot of stuns or slows. You won't be able to know who you are against unless you are in Draft Mode, so in my opinion, Cleanse is pointless.


What not to get:

Clarity: Let the casters use that. Besides mana manipulator/soul shroud should fulfill almost all your mana needs.
Revive: This is a totally useless spell (unless you are trying to break the speed record).
Teleport: You have Recall and Grand Skyfall. You really don't need this unless you back too much (Note: Because of the last patch, I recommend this more than say Revive, but still less than heal/flash/etc.)
Smite: This is not a jungling build. Do not get it.

Use Spear Shot to get last hits on minions if you are against harassers. With a <4 cooldown, you are guaranteed to get most minion kills. You can also harass due to this.
Farming is EXTREMELY important as many of the items are high-cost.

Pros:

1. Pantheon is an initiator. Why not put some health on him to keep him from instant death?
2. Most people don't expect Pantheon to go tanky as he is made to be dps.
3. You can still do a lot of damage as soon as you get Atma's Impaler.
4. This can be a very fun build!

Cons:

1. It takes a while to get rolling and used to this build
2. You don't do a lot of damage until late game or if the game finishes before Atma's, never.
3. Madred's Bloodrazor will own you. Period.

As I said before, Pantheon is an initiator. You need to be either the first or second person into battle. Use Aegis of Zeonia to stun whoever will do the most damage or is the lowest on health. Have your teammates target whoever you stun, otherwise your jump went to waste.

Use your ult to get around where possible. With this last patch, your ult now has a limited range (which is really gay in my opinion. It was supposed to be a global ult, now it is a s***** ult because you can't get where you need to sometimes. This is all because of the stupid limit).

Remember this is a fun build. Please no trolling, but feel free to give comments and suggestions. It does works, and as soon as I can, I will attach a picture proving it.