Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
I thought that when you are in iTunes you can just drag the playlist from the iPhone into iTunes? Just expand the list.
 
Go to the first page, and check 'Manually mange music" or whatever it is. Then try it again. Should work.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.