If you where lucky enough to get hold of a copy of iOS5 Golden Master prior to today’s release, you may find now when you try an download the ‘public’ release, iTunes tells you your already running the latest version!
However, try not to panic too much! All you need to do is make sure you have a recent backup, then choose the restore option in iTunes. This will whizz off to iTunes update servers and download the latest version of iOS5, flatten your device then install!
Then all you need to do, is restore your backup – presumably from your iCloud accounts!