Okay, here's some info.
Release date is February 2010
It is avalible on the PS3, 360, Wii, DS and PC.
The characters confirmed so far are:
Sonic (Duh)
Tails
Knuckles
Eggman
Amy
Shadow
Big
Billy Hatcher
AiAi
Amigo
Ryo Hazuki
Beat
Alex Kidd
Zobio and Zobiko
Confirmed courses:
Seaside Hill
Casino Park
Final Fortress
Blizzard Castle
Curien Mansion
Tokyo-to
(There are 2 other courses but they have not been given names, a track from Samba Di Amigo and another from Super Monkey Ball)
Every character has their own special move, the effect depending on your character.
The game will have up to 8 players online in each race.