Earlier this week, Apple started disabling all "non-developer" devices that are running iOS 5 betas - registered to a developer account. Theoretically, there is no way that Apple could know if a device running iOS 5 is not being used by a developer. Apple provides 100 UDID slots to developers so they can beta test their upcoming software with many iOS device users. Apple does not ask developers to specify which UDID actually belongs to the developers.
Howerver it looks, Apple has identified those developers who sold their slots for UDIDs. According to report, "many developers have reported that Apple has sent an email warning which said that they have identified the developer to be selling his slots for some users to get early preview of iOS. And, then Apple has started closing the developer accounts for selling the slots and also, have flagged the UDIDs associated with that dev account, thus making the iOS 5 device unusable.".
A developers whose account was recently shut down due to their selling of UDID slots states:
My iOS developer account was disabled by Apple recently, last month, as it had appeared that I was breaking the agreement (iOS PLA) by distributing Apple confidential data. Granted, I was selling UDID slots.