Relmstein has a good post up about how Blizzard's world PvP has evolved over time, and what we might expect from Lake Wintergrasp, the PvP zone in Wrath of the Lich King. When world PvP began, it was very much an ad-hoc environment, which lead to a lot of 1v1 battles that eventually escalated when wandering guildies or alts showed up. As Relmstein says, the large majority of battles started with that stealthing noise (that used to inspire chills in the spines of those on PvP servers), and ended with an all-out brawl with no rewards but to get the other guys back.
Nowadays, world PvP is much more organized, in the sense that there are specific objectives and goals to go after. The original reward was a zonewide buff (in Silithus and the Plaguelands), but Blizzard eventually morphed this into something more permanent -- in Auchindoin, you fight for a buff that earns you tokens for rewards, and in Halaa, you fight to get the rewards themselves.
Which leads Relmstein to suspect that Wintergrasp will have direct rewards of its own, almost equal to the gear you get from endgame dungeons and Arena PvP. He predicts that it will be token-based (not a bad idea, considering all the badge changes Blizzard has put in lately -- in fact, Blizzard might even cut a corner and have Wintergrasp reward players with WotLK's version of badges directly), and that it will be updated with each Arena season, to keep players fighting there. All good ideas -- Blizzard has definitely innovated (successfully or otherwise) in the area of World PvP rewards, and we'll look forward to Wrath to see what they come up with there.