Previously this wasn't possible with the iPhone, only iPods, but it's finally here! As long as you manually mange your iPhone's music, you'll be able to drag and drop music from multiple computers to your phone. My iPhone is synced with my MacBook Pro and I just added a song to it from my PC. I reconnected it to my MBP, and hit sync to see if it will be added to my MBP's library. It has been backing up for about 10 minutes now. Not sure what that's about, we'll see. This is with iTunes 9 and iPhone 3.1. Also, with Home Sharing you can now transfer files between two libraries but unfortunately play count STILL doesn't update on another computer when you play a file from it over a network. Just thought I'd share this.