Banana Shoulders has a really excellent guide to just what the Argent Tournament is all about. I played it for the first time the other night (while we were streaming live from the PTR), and it's a strange little mix of minigame, questing, and rep grind. The Argent Tournament has set up north of Sindragosa's Fall, and there are three basic stages: first, we've got to help them build the place (which will send you to quest in other parts of the world), second, you'll be moving up the ranks from Aspirant in the tournament to Champion, and third, you'll be acting as an Argent Tournament Champion, and picking up Champion's Seals to spend on various rewards and items.
The tournament itself involves a minigame of mounted combat -- you can ride around on a mount, lance equipped, that has a few different abilities. There's a ranged attack that will let you break down an opponent's shield, there's a Defend ability that will let you build up your own shield, and then there's a Thrust melee attack that just does damage, and a Charge attack that will do damage as well as break down your opponent's shield. And out of combat, you can fully heal your mount or challenge someone else to a duel. The Aspirant quests teach you all of this, and then the Valiant quests really put your knowledge to the test as you move up through the ranks, fighting NPC mobs as challengers to the title.
The Argent Tournament definitely seems like a lot of fun -- as I said way back when, the whole event seems like a one-stop shop for picking up rep and items in Northrend, so I wouldn't be surprised at all if Blizzard plans to add more events and more daily quests on to what we're building up there. Banana Shoulders will be posting part three of their guide soon, and of course you can stay tuned to WoW Insider -- as we get closer to the patch 3.1 release on the live realms, we'll have everything you need to know about the Tournament and then some.