A recent post on the gaming forum NeoGaf has possibly revealed info on the Wii U’s specs, particularly its resolutions.
Game resolution will mostly be at 720p, with some simple first party games and software, such as Mii games, going up to 1080p HD. This would place the Wii U above the Xbox 360 and PS3, since the PS3 runs most of its games at below 720p (although settings are adjustable). Wii U games would run higher, but the tablet controller makes the system work harder since it runs on 480p by itself.
There aren’t really any reliable sources since this is all coming from just one forum poster, but leaked video game information isn’t all that uncommon and it has to come from somewhere. They seem like reasonable and nice specs to me.