Specifically, iTunes would conk out about three-quarters of the way into backing up the iPhone. I literally waited hours, hoping it would eventually finish the process, but no luck.
I won't bore you with the gory details, other than to say I did a lot of resetting, restoring, waiting around, cursing, and hair-pulling, all to no avail. (I hope it comforts you to know that even we so-called experts run into crazy glitches like this.)
One thing I tried was booting my Vista partition and syncing the iPhone there. And guess what? It worked! That, my friends, is exactly why I'm a proponent of making a gradual migration to Windows 7: If something doesn't work right, you have a fallback.
Still, I needed my iPhone to sync in Windows 7. But no amount of troubleshooting would solve the problem.
Then, I tried something totally crazy: I plugged the sync cable into a different USB port. And believe it or not, that did the trick. The iPhone zipped through the backup process and was back to normal. Hallelujah.
I can't say this will solve whatever sync problems you might be experiencing, but it's definitely worth a try. (Just make sure you're not in the middle of syncing when you unplug the cable.)