Interestingly, this is the first version of iOS5 that is said to be able to be applied OTA -- though that particular option comes with the warning that all photos will be deleted should you give it a try. We're guessing that "feature" will not be present in the final release candidate, but we could be wrong. Would certainly make for a fun Easter egg...
Update: We're actually having issues applying this on our many and myriad devices, and we're seeing others reporting the same. We'll keep trying, but let us know how you're getting on in comments.
Update 2: Looks like we got it. We had to go back to 4.3.4 and then we were able to move up to beta 4. Maybe that'll work for you. Maybe you'll be still stuck in the doldrums of beta 3. Either way, you're still okay by our book.
Update 3: Looks like the jailbreak is out in full force. Not too shabby, eh?