iphone only backs up but won't update

I am trying to update my iphone 4 to ios 5.1. It tells me there is an update available and so i hit "update". Then it tells me itunes will backs up my phone and then install the update. It takes an hour while it says it's backing up. Then it just says sync is complete. It hasn't updated anything though. It's like it's only backing up. I've tried this a bunch of times and it still does the same thing. No error message or anything. Afterwards it still just says that an update is available.