I was not expecting another demo, but SEGA just announced that another one is coming very soon.
The new demo will be out on October 19 on XBLA and October 25 on the PSN in the US, and on October 19 for both systems in Europe. It will not only contain the classic Green Hill Sonic Generations level from the first one, but also add the modern equivalent of the Green Hill Zone. As a fan of the Sonic Adventures titles, this is nothing but good news.
The game will be released on the 4th of November on the PS3, Xbox360 and digitally on PC, with 3DS and boxed PC versions coming later.