Dennis, from a German gaming magazine, has confirmed that Sonic Unleashed will indeed be a multiplatform release:
“Sonic unleashed will be on the cover of the next PLAY magazine, it will also feature in Nintendo’s magazine (NOM) and will be in the August issue. The screens I have uploaded are from the Xbox 360 version of the game, however, the game is going to be a multiformat release.”
This confirms both a 360 and a Wii version, at least.
Special thanks goes out to TSS’s own Casanova for reporting this info, and sending in the above Google translation.