I had this problem. In my case, I selected "Transfer Purchases from iPod" and iTunes would begin to transfer them and then - nothing. The same happened when I right clicked the iPhone and selected the same Transfer... option. It would start, and immediately stop.

I resolved this problem by unclicking the "Prevent iPods and iPhones from syncing automatically" checkbox in the Devices section of iTunes' preferences. (For the record, this is unchecked by default. I checked it to avoid auto-syncing confusion with friends iPods, etc.)

I hope this helps...