One of these days I hope they'll add the ability for me to sync my iPhone to iTunes at home and listen to it on another PC (most specifically the office) from my iPhone via iTunes without some horrible hack and manually managing my music collection.
You can't copy the files off (without some 3rd party software) but I thought you could browse and play via iTunes. I remember it being clunky enough that you might not want to do it regularly but I thought it worked for playing a couple of tunes.
I actually just stopped putting music on my iPhone because it was too much of a hassle to do every time that key changed. I just use Pandora or GrooveShark now instead.