Simplify Media available in the App Store now, allowing streaming of iTunes music library to your iPhone.  The official app has album art, lyrics and artist bios and works over EDGE, 3G and Wi-FI.

It's free for the first 100k downloaders:
http://phobos.apple.com/WebObjects/MZStore.woa/wa/viewSoftware?id=284941327&mt=8