i guess you need to download the latest itune and try again ..coz mine also having the same problem ... but after i re-install the itune and it back to normal again..

This happened to my Nano but I haven't had this problem with the iPhone. What I did was I disabled the iPod service from My Computer>rightclick>Manage>Services and Application>Services , plugged in my Nano and enabled it again and it worked. Not sure if it's the same for the iPhone.
After I reformatted my computer (not because of this problem, because PC's just need a clean sweep every now and then), the problem disappeared so I never found out what was actually wrong..