When Rayman Legends launches as a Wii U game this fall, that's all it'll be launching as. According to the last three or so seconds of the game's Gamescom trailer above, Rayman Legends will be a Wii U exclusive.
Considering the amount of touchscreen/Wiipad stuff going in this trailer, it's clear that Ubisoft has been taking Wii U integration seriously – we just didn't think it was that serious. All Rayman titles up until this point have been multiplatform, and since Ubisoft itself said the game wasn't necessarily an exclusive back in June, we kinda expected that to be the case.
We asked Ubisoft for clarification just to make super extra sure, and were told that "yes, Rayman Legends is a Wii U exclusive. It will be releasing in Holiday 2012."
Still, we'd be surprised if the game didn't end up other other platforms eventually, though we imagine quite a bit of rejiggering would be required to make Rayman Legends function on an Xbox. The PS3, however, could handle it quite easily provided there was also a Vita involved.