This problem is quite common, but I have not been able to figure out what causes it. My best bet is that it's a USB issue, and I have found a workaround that makes synchronising possible when all else fails. For the record, I have tried all of the potential solutions on these forums without success. Here's what worked for me.
(Windows 7 32bit, iPhone 4S, iTunes 10.6.1.7)
First of all, make sure you have a good wi-fi signal in the region of the PC you are syncing with. Then -
1) Start iTunes.
2) Connect phone via USB.
3) Left click on phone in iTunes.
4) On the Summary page check "Sync with this phone over Wi Fi"
5) On the phone go to Settings>General>iTunes wi-fi sync
6) Tap on Sync now.
The phone should sync via wi-fi, even though it is connected via USB. It didn't seem any slower to me than syncing via the cable, and more importantly it worked, and the backup went smoothly. I've tried it several times now with no problems.
Also, if you have a lot of apps you need to synchronise or you are re-arranging the app windows using iTunes you can sync in the normal way from the PC if you do it immediately after the wi-fi sync, because iTunes does not run the backup again. It probably won't run two backups within a few minutes of one another.
It's all a bit of a fiddle, and Apple/MS needs to get a fix for it, but if this method works for you at least you have a good backup should you flush your phone down the toilet!
Good luck!
