It definitely does *not* use the same GPU as the Gamecube. ATI didn't split the engineering teams of that graphics chip and the one for the 360 because it was going to use Flipper.

Both the Wii and 360 graphics chips are based on what WAS to become R500 from ATI. The retail R500 design was pulled from retail development because they sold Nintendo and Microsoft on the designs. That's also the reason the retail R5XX ended up still being based on R300 architecture (the original R400 had already been scrapped in favor of jumping straight to R500 - ooops! and was also replaced with an R300 derivative called R420).

That said, I don't know how the final chip design panned out and how many gates were chopped/added to either hunk of silicon that went to MS and Nintendo. BTW, the only reason the Playstation 3 doesn't have an ATI chip in it is because ATI was allready in bed with MS and Nintendo. Sony had to go with Nvidia because they were simply the only option left after the Cell was found to be completely broken in the graphics department.

Ok, TMI...