I saw this posted on another board. It's just a rumor for now (time will tell), but it sounds like it'd be kinda cool. Click here for details. Here are the tweets from Nsider_mag:
What would you think about this?
We've just had some shocking news from an inside source at Nintendo: Sonic will be a playable character in Mario Galaxy 2! Wha?
More teasing from our insider: Sonic levels will be in 'Green Hill Galaxy'. Matthew's nearly in tears (of despair)
What would you think about this?