The latest character select screen has Miis as selectable characters...

It seems that Mario & Sonic at the Olympic Games will have support for Miis.

The character select screen in a playable demo of Mario & Sonic at the Olympic Games for the Wii Connection Tour in Australia has the option to select a Mii. It's position on the secect screen suggest that you can use your Miis to race against Sonic and jump against Mario.

Thanks to GameArena

Mario & Sonic at the Olympic Games is due for release in Europe on the 23rd November 2007.