We will be performing a server-side data wipe on all CloudKit public and private databases for iOS 8 beta and OS X Yosemite Developer Preview users on July 7, 2014. The following iCloud features will be affected: iCloud Drive, iCloud Photo Library, and Mail Drop.
Photos and videos stored in iCloud Photo Library will remain on their original iOS 8 devices and will upload to iCloud Photo Library again automatically. iCloud Drive can be re-enabled from Set up Assistant after upgrade. If you choose to store your documents in iCloud Drive, your Documents & Data will automatically be copied to iCloud Drive. iCloud Drive will not update across earlier seeds or operating systems. Attachments sent through Mail Drop will expire and need to be resent after you upgrade.
If you have any questions, visit the Apple Developer Forums.
Apple Developer Technical Support