Sega has announced in its official blog that new downloadable content would be coming to the PC version of Sonic Generations on the 26th December.
The "Casino Night Pinball" DLC will include a pinball mini game, set in the Casino Night stage, to enjoy alongside the main game. The DLC will cost USD $1.59, GBP £0.99, EUR €1.59, AUD $2.99.
Sonic Generations was released on the PC, Xbox 360 and PlayStation 3 on the 4th November. The blog did not make any mention of a console version of the DLC, though it's possible that it could be undergoing certification.