I copied two files onto my jailbroken iPhone 4 (from a 2G AppleTV, as detailed here:)

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

And now, my iPhone now shows up as a destination speaker in in iTunes' Airplay pulldown and dialog window. Love it.

The even better part? It shows up as a destination speaker in Airfoil, too! It's as if I have native, built-in version if Airfoil Speakers Touch.

Check it out. Very cool stuff. If you're jailbroken you probably also have SBSettings installed. Use this hack in conjunction with the "Autolock" toggle, so you can set your iPhone to not shut off while it's receiving your audio stream.