Mike Ter Louw has brilliantly managed to port mt-daapd, the Firefly Media Server, to iPhone. Firefly makes use of your iPhone's built in mDNS/Bonjour media service to stream audio to iTunes. Any iTunes. The one running on your home computer. The one running on a friend's computer. Wherever. Stream from your iPhone to a quality audio system. Here's how it works.
Navigate to http://home.mike.tl/iphone/ with Mobile Safari. This adds Mike's repository to Installer.app. Then launch Installer, select Firefly and install it. Wait as the package builds your music database. This can take 5-10 minutes based on the size of your iTunes library.
On your computer--or any computer you're visiting with--set iTunes to share music. Make sure your firewall settings allow communication and enable "Look for shared library" in your iTunes Sharing preferences.
In iTunes, locate your shared library--it will be listed by the name of your iPhone. Select it and log in. Even if you're using 1.1.1, the media password still appears to be "dottie".
Then just select a song and play! This, ahem, rocks. A lot.
Update: TUAW Reader Jonathan adds: "Safari redirects to /beta/. I bypassed that by going into installer.app's sources page and manually adding http://home.mike.tl/iphone. Installer picked it up right away, and it's building my music database now. "