Simplify Media's just released a new version of their Mobile suite for iPhone that lets you stream iTunes libraries from your friends across the internet. It works in much the same way the desktop version does. Load up Simplify Media on a desktop with a library that you want to share, then log into your Simplify Media Mobile on your iPhone (with Wi-Fi) and you can stream all of your and your friends' music back over the air.

If you really want to stream on the go, without Wi-Fi, you could probably rig up a Wi-Fi internet connection sharing thing with a Windows Mobile phone as well.

We tested it over Wi-Fi and it streamed back all of Adam Pash's songs without a problem (eventually). It crashed and hung the first couple times. [Simplify Media]