If you’re having this problem then you’re not alone. A quick look on the Apple forums will tell you that there is a problem with the sync getting stuck either on step4 or 4 if you’re trying to sync by wireless, or step 6 of 6 is you’re plugged in.
Read through the forum replies and you’ll see lots of advice on things to try. However, what worked for me is to simply restore the phone in iTunes. This reinstalls iOS5 onto your phone along with the up to date firmware. Then it rebuilds your applications and music etc from your last backup.
What’s interesting during the restore process is that it now appears to rebuild your content from a manifest (a list of what should be installed on your phone) as opposed to restoring from a backup.
I might be wrong but I think this is new, and the benefit of doing it like this is it clears your phone down completely and rebuilds it with the most up to date applications.