Wii hardware was capable of playing GameCube discs, but that won't be possible in Wii U. Since it's compatible with Wii remotes and accesories, it'll obviously play Wii discs, but the question is whether Nintendo will alter the look of those Wii games with the more powerful Wii U.
The answer is no, based on a conversation I had yesterday with Mark Franklin, head of Nintendo's public relations. Pop in a Wii disc, the games will play like they did on the Wii.
No upscaling or texture filtering, unfortunately.
Ah well. The dream of effortlessly getting Super Mario Galaxy 2 in HD has been shot down.