I just wanted to jump in and mention that the Wii has double the clock speed of a GC and a better gfx coproc. The GC could put out 480p. I don't think it's that big of a jump to think that the Wii could do more.
Nintnendo is a small company in relative terms. They also develop a lot of first-party titles. As such, they really have to divide their resources strategically. When you think about all the different features of the Wii and all the more pressing concerns (online gaming, downloadable content, etc.), it seems, at least to me, that it would be awhile before Nintendo supported HD.
But that doesn't mean the Wii can't do it. It was mentioned that the Xbox did 720. That was an 800 MHz P-III...
I've got my component cables on order from nintendo.com, and I'm stteing up my new 27" LCD HDTV this afternoon, so I think Nintendo did a good job of at least putting 480p out initially.