iOS 4.1 has new encryption technology, that will constantly check if iPhone or iPod touch, have been jailbroken. This is done by digital watermark that appears when iOS is jailbroken, and this watermark cannot be removed, even if reverted to unjailbroken status. Apple store employees will check for the watermark during iPhone service.
via iOS 4.1 (Beta 1) Now Available Worldwide. Come See What’s New! – [Updated x13].