Sonic 4 Episode 1 is available at the AppStore (9,99$). First reviews are positive, and report a smooth framerate, even on older iPhone 3G. However the game is said to be a somewhat short. It does not provide retina display on iPhone 4. We can watch a trailer here.
We can also find Modern Combat 2: Black Pegasus (6,99$), that supports retina display. It features 12 single player levels, and an online multiplayer game.