I know it's in French but this article is actually an interview relating to the game:
http://www.gamekult.com/articles/A0000051320/. Further down, it reveals that 8 players will be playable in Multiplayer games! I can translate the rest and edit it here.
EDIT:
Questions in context (paraphrasing) ask:
-What's it like developing a game for the Wii?
-How does Sonic and the Secret Rings work, exactly? (controls)
-Isn't this kinda simple?
-Given the apparent simplicity of the game, how long do you think the game will last?
-Is this because of previous games in the series; will other characters will be playable?
-Will online capabilities be possible?
EDIT 2:
Interesting points of the interview:
-Implemented 30 mini-games
-4 simultaneous multiplayer capabilities that will use frequent usages of the... controller (?)
-One mini-game involves players choosing a chest (said trunk in translation but I don't think that's what they meant. :P), stirring it in all directions it directs to get the item it contains (something along those lines).
-About ten hours to complete
-70 Missions
-Quite a few unlockables for eager gamers; gamers can try to attain high scores and times; gamers can discover hidden items and other secret bonuses (Personal thought: wonder if special stage will be implemented? That would be so cool!)
-Made the game simplistic to minimize complicated maneuvers (all of you can easily recall Riders' control mechanism; personally, had no problem with it but a lot of you did) giving it a mixture between a RACE game and an ACTION game.
-Like I said above, there are 8 possible/different "heroes" in/for the mini-games
-If time allows, would like to feature some kind of Wi-Fi connection/online capabilities