According to vg247, Sega has released the demo for Sonic Generations on the XBOX Live Marketplace and Playstation Network Store as a Gold Exclusive. It weighs in at 652 mb for North America and 664 Mb on Europe’s PSN Store. Reportedly the demo contains Act 1 of the Green Hill Zone and you play as “Chubby Sonic”. Sonic Generations releases in November 2011 on the 3DS and probably around the same time for PS3 and the XBOX 360.
Also today is Sonic’s 20th Anniversary so I expect we’ll see more news from Sega as their main man Sonic celebrates the big 2-0.