Users that would like a sneak peek at beta iOS builds would normally have to enroll in Apple's iOS Developer Program, which costs $99 a year. By doing so, they can register their device's Universally Unique Identifier (UUID), which will authorize their device for access to iOS betas.
Up until recently, users who chose to get around paying the annual $99 fee had an option of going through various third parties available off Google searches, who advertise registering UUIDs for a small fee - one site offers this service for 10 GBP.
However, a recent report cited by AppleInsider suggests that some of these users who went through unauthorized means to get beta iOS access are getting locked out of their devices. In addition, Apple has reportedly began closing down accounts that were used to sell "device slots" via the Internet. According to the iOS Developer Program rules, developers are prohibited from allowing non-developers access to iOS betas, in addition to making a profit off device slot sales.
The report describes the behavior of locked out devices as rebooting to the initial setup screen, whereby users are prompted with the selection of a WiFi network. Following that, the device freezes at a black screen.
The sudden lockout of unauthorized users may be a timely coincidence, as Mac Rumors points out the majority of the locked out devices were running Beta 1 and 2 of iOS, which have expired. A few commenters to Mac Rumors' article have reported lockouts on Beta 3 and 4, but it is not yet known if the beta expiration is the root cause of the lockout, or Apple's new crackdown.
Users who are wary of device lockouts should wait until the public release of iOS 5, which is set for this fall. The new operating system is slated to be released alongside the new iPhone 5.