They said it was coming, and now it has come. Sonic Spinball is available on RealPlayer's RealOne Arcade service starting today.
You can download it for free, and you get a one-hour trial where you can tackle the Veg-O-Fortress to your heart's content... and if you want to keep playing after that hour, it will cost you $14.99 for the full version.]
According to SEGA's RealOne page, Sonic 3D is also coming later this month. So stay tuned for that.