My suggestions in order of severity of problem:

1. Try force-quitting iTunes; don’t unplug the phone before you do-worst case you will lose anything prior to last time you backed up/sync’d successfully. If that works in stopping the sync, then un-dock the iPhone and restart your computer, then try to sync again.

2. If that doesn’t solve the problem the problem may lie with the iPhone’s memory, or the OS/firmware; Try rebooting the iPhone by holding down the home and lock buttons simultaneously until the iPhone turns off then shows the Apple logo; once it has rebooted and returned to the standby screen, then try re-docking and try a re-sync; if that doesn’t work, then:

3. Try resetting/restoring the iPhone’s OS from iTunes; dock your iPhone then click the Restore button to re-install the OS (note-you’ll lose everything on the iPhone that was added since the last successful sync); Also if you have a jailbroken iPhone it will re-lock the phone; if that doesn’t work then:

4. iTunes needs an update; try updating the software (on a mac-Apple menu/software update); then try re-sync after restart;

If all else fails, then contact Apple…