Apple has started testing iOS 5 Beta couple of months ago and so far 3 revisions have been released for Developers to test and debug. But having iOS 5 Beta being a non-developer was a charm since day one of release as you have to pay a hefty 100 US$ to become Apple beta tester. So there were various methods for getting iOS 5 Beta as a non-developer and the most famous among them was to get UDID (Universal Device Identification) slot form a person having developers account. Therefore by grabbing a UDID for iOS 5 you also become party to install iOS 5. However this action of Apple has turned this game over for non-developers to use iOS 5.
Appleinsider reported that Apple has already identified the activity going on and has started taking action against those developers selling UDID. Firms which were selling UDIDs have got termination notification from Apple for selling UDIDs. Once your device is flagged by Apple using UDID registration you are forced to initial setup meaning you no longer able to use iOS 5 either. One of such firm claims that they have registered almost 15k UDIDs since 2009 for all the iOS Betas launched in this time frame.
Although this information can not be verified at the moment but still to be on safer side if you have upgraded your device to iOS 5 Beta try downgrading to iOS 4 using procedure here.