SEGA is celebrating Sonic the Hedgehog’s 20th Anniversary with Sonic Generations which sees the Blue Blur playable as his 1991 guise and his more modern incarnation. The game will be out on Xbox 360 and PS3 in time for December 2011.
The blurb tells us that the game is "Set across three defining eras from 20 years of Sonic the Hedgehog video game history". The environments have been re-created in HD and are "now playable in both classic side scrolling 2D from 1991, as well as modern 3D style found in Sonic’s most recent adventures."