iPhone as AirTunes receiver
If you are willing to jailbreak your iPhone there is an app on Cydia that will turn your iPhone/iPod/iPad into an AirPlay speaker. It has its issues, but it works fairly well.

If you have want to mess with it there is an article here:

http://www.macstories.net/tutorials/how-to-turn-your-iphone-and-ipad-into-airplay-speakers/