9to5Mac hears that the iPhone and iPod touch are getting a radio app like the nano's that'll be able play FM radio in the background. It'll be integrated with the iTunes store, supposedly, so you can buy songs you like.
The hardware capability for FM radio does exists in some models—the current iPod touch and iPhone 3GS have FM transmitters, and the 2nd-gen iPod touch also has a chip that's capable of receiving FM signals, though it uses it for Nike+ stuff (so if there is a radio app, maybe it'll just be for current-gen models).
It'd make sense to add more software parity across the line, but who knows—no date given for when we might actually see this FM radio app. [9to5Mac}