Here's the fourth official trailer for Sonic Generations.
This one shows off three levels, Seaside Hill/Ocean Palace from Sonic Heroes, Spagonia from Unleashed, and Chemical Plant Zone from Sonic 2. This one even gives some insight about the story, basically just telling us there's some time travel involved. And apparently future Tails went with Sonic back in time too.
Also, the game comes out Nov 1, 2011 and pre-ordering with Gamestop will give you a downloadable level (Casino Night Pinball Stage), an Xbox dashboard/PSN Home theme, and for Xbox 360 owners, a downloadable avatar item (Super Sonic Costume).