AppleInsider, along with Macrumors, have both received reports from various people that Apple is now actively disabling UDID slots for use in iOS 5 due to developers taking advantage of the system and selling the slots off for a profit. This results in the user of such a device running iOS 5 with an illegitimately activated UDID encountering the same symptoms which happen when a beta expires (asks for your iCloud login information, requires you to go through setup again, etc.). We have seen a few threads started in our forums based around this, so it would seem that the reports received by Macrumors and AppleInsider are accurate.
If you are someone who has had their UDID activated through one of these websites, it is probably best if you simply downgrade back to the latest official iOS version (currently iOS 4.3.5). This can be achieved by going in to DFU mode and clicking on “Restore” in iTunes while holding the “Shift” key (or “Command”, on OS X). You also may be able to simply hit “Restore”, though that will probably be hit and miss.
The reports are still coming in, and this is still unconfirmed by Apple. However, it does seem likely that Apple is (finally?) taking a move against developers selling slots to non-developers.