9to5mac reports they've got a source who says this exact app is in the works for the iPhone. Supposedly we're all waiting on the integration of MobileMusicStore (iTunes.app) with this - basically, if you're listening to a station in the US that supports tagging, you'll be listening to a song, tap an info button or something to grab all the info on that song, and be able to click to purchase it straight from your iPhone/iPod Touch.
The iPhone/iPod Touch have been able to receive FM for a while now - it's how they talk to the Nike+ line of products. Turning that on for device owners to listen to the radio would be great, and also a ton of additional revenue for Apple and artists. Or at least Apple and labels.
Their source mentions iPod.app might even see some FM functionality before the release of Radio.app proper.