No, the game will not include Sonic’s first Sonic Adventure.
Sonic Generations looks to be an interesting experiment for SEGA. Taking bits from each era of Sonic gameplay (Genesis, “Dreamcast” [which includes games never on a Dreamcast], and Modern) and Sonic Jamming them together, it looks to sate all sorts of of fandom. Those who like side-scrolling Sonic will get to play their way, while those who like the head-forward approach of modern Sonic can get their action on.
Really, though, the Genesis era was the series of inarguable successes. With all four titles available on Xbox Live, PlayStation Network, and the Wii’s Virtual Console (and Sonic CD headed to the former two this fall), chances are you’ve played the original titles. SEGA, looking to increase the value of Sonic Generations, have decided to put a free copy of Sonic the Hedgehog as an unlockable title in Generations. There’s no mention of how to unlock it, but chances are you’d have to beat the game.
Have you somehow never played the original title?