Sonic 4: Episode 1 to run at 1080p
If you buy them on Xbox 360 and Playstation 3 of course. This is backed up by both product pages on PSN and XBLA.
No idea of this is native resolution or upscaled. I have decided to drop SEGA an e-mail and wait like a rabid dog all weekend for a response. I hope the answer is yes.
[Source: Sonic Stadium]