From what I've seen elsewhere, it's purely upscaling, not rendering the games at higher resolution than before.KDub wrote: Wii games do actually play in 1080p, as do all the menus and anything else. This was a big surprise! I played Goldeneye for a bit and you can see a difference, both good and bad. Some things in game look much smoother from the upscale but mistakes or ugly filters stand out more. The cut scenes looked really gross though! This is because it is an upscale of resolution but no re-rendering is happening, no new filter or anything. This isn't like emulation where new rendering processes can be applied; it is just a resolution increase. So you’ll get good things and bad things from playing Wii games on WiiU, it probably all depends on the game.
Realistically, unless you're playing in a little box in the middle of the screen, something is upscaling the picture to an HDTV's native resolution. Apparently the Wii U just does a nice job of it internally, at least probably better than a lot of low/mid level AV gear.
IGN did a side by side video here for example.