ok here wii can play:
1. Wii games
2. GC games
3. Downloaded games (NES, SNES, N64 and more
1. Wii games have the abbility to use the wiimote and possibly (nothing confirmed yet) gamecube controllers, mainly the wiimotes motion features.
2. gamecube games with the gamecube controller there are ports on the top for 4 GC controller slots, and there is two slots for GC mem cards...
3. downloaded games from the wiishop channel where you BUY older games with wiipoint cards you get from stores or with a credit card through the nintendo wii, and you can only play these games with the classic controller
i hope this helps settle some confusion

EDIT
forgot, the ability to download games for the virtual console part of the wii (via the wiishop channel) IS available at launch according to nintendo there will be i think 12 games? dont remember how many were available at launch coming with more in december