If you've played the recent dominion beta it's unlikely you haven't figured out that Rammus is incredibly powerful. Just building him as you do on the rift makes you formidable based on your bulkiness and ability to be where the team needs you. But the players who have been making most impact with Rammus, to my observation, build very differently than on Summoner's Rift. The Rammus build that seems to have most impact is a capture Rammus.
The first question that arises is "Why Rammus?" and the answer is quite simply his powerball makes him inarguable the most mobile champion available. His high base defenses also contribute by making it so that he is resilient enough to get away without heavy item investments. The only other champions I consider potentially rivaling him for this role are Teemo and Kassadin.
The next question is: "What exactly do you mean by capture Rammus, and why is this role so powerful?" By capture Rammus, I mean that your job is ONLY to neutralize or capture control points. You aren't going to fight. You have two jobs: Capture unattended points to which the enemy can't respond quickly, and to provide emergency disruption of enemy captures on your towers. As for why this role is important, and why can't we simply have 3 man pushes on towers, is because having a roaming Rammus capturing points disrupts the enemy team.
Two good things happen when a Rammus quick-neutralizes/captures a point, first, someone is forced to respond. This makes the other fights swing in your teams favor, since the other team is now a few men short. Second, it creates a minor point swing. That minor point swing, perhaps 2-3 points for each tower you capture, stacks up to decide those close games that seem all too frequent.

Pros:

VERY high overall scores
Easy escapes
Huge amount of captures
Moderate tankiness

Cons:

Squishier than more tank-centric Rammi
You're going to die reasonably often

Now the item build is probably incredibly outlandish to anyone who plays Rammus or another tank with any amount of frequency. You get barely any health early on, and no armor. Why build a Rammus like this. The answer should be obvious: Speed.

Boots of mobility have become standard on any dominion Rammus, simply because they have a huge impact early game, and combined with powerball, they do stupid things.

Priscilla's Blessing is actually a recommended Rammus item, and Riot did good in making it so. You will underestimate the impact of +15% capture rate right up until you realize just how much faster you can steal points. And the faster you do that, the faster you can get out of there to avoid incoming defenders. The active is also a beauty. It gives you a quick boost for getting to that undefended turret faster.

Shurelya's Reverie is a item along the same vein as Priscilla's. The speed boost is crucial, and helps teammates as well. It should usually be used whenever it's off cooldown if you're powerballing somewhere. The kindlegem in it gives you a bit of health to make up for the health your not building, and lets you spam abilities a bit more liberally.

Force of Nature is pretty odd to grab before you have high health, but there are reasons for it. First, you have so much health regeneration already that more would make returning to base unnecessary. Second, it's gives you a speed boost on top of the regeneration. Sure, it's only a Zeal's worth, but the health regeneration is incredible in it's own right.

Warmog's Armor seems a bit more sane than the rest of this build. It gives force of nature a much bigger purpose, it soaks up the increased damage floating around about now in the game, and gives you YET MORE regeneration. Most importantly, about this point in the game is when the enemy defends more intently. As such, you're going to want to be thicker for the eventual ambush.

Phantom Dancer is just a personal choice. There are probably a billion better options, but I like phantom dancer for it's raw speed boost. Usually at this point in the game it's frantic and getting there that much faster matters that much more.

More coming soon.