Now that iTunes 10.5, iOS5 and the new Xcode have been released, it is worth mentioning that to uninstall any beta installation you had, you should use the uninstaller.
sudo <Xcode>/Library/uninstall-devtools --mode=all
Where <Xcode> is the directory where you installed the beta Xcode.
I usually install the different versions into different directories (/Developer, /Developer5b3 etc).