The "Casino Night Pinball" minigame that console gamers got for preordering the game will be available to PC gamers via Steam on Dec. 26. It'll only run you $US1.59 in the U.S. (Overseas, it's £0.99, €1.59, $US2.99 in Australia).
In the minigame, you play as either classic Sonic or modern Sonic, and he's the pinball. Modern Sonic has a limited number of boosts, but otherwise the two are identical. Sega released these three screenshots with the announcement.
Still no word on when or if this will be made available over Xbox Live or PlayStation Network.