Nintendo and SEGA announced today (Tokyo) that both Mario and Sonic would be making a debut in the Mario & Sonic at the Olympic Games which is based on the Beijing Olympic Games coming up in 2008. Other characters making a debut are Luigi, Yoshi, Knuckles, and Tails besides Mario and Sonic. There are others but the full list was not mentioned in the press room.
This game has been slated for a "Holiday 2007" release.
UPDATE: Sorry guys I forgot to say the game will be hitting Wii and DS platforms.