As pointed out in the OS X Yosemite seed note issued on Monday, Apple will perform a scheduled wipe of CloudKit databases tomorrow. The first data dump came earlier in July, affecting both public and private iCloud data connected to the current iOS 8 and Yosemite beta builds.
Apple did not provide further information regarding the upcoming action, though the initial server-side CloudKit wipe affected iCloud Drive, iCloud Photo Library and Mail Drop attachments.
After the most recent expungement, Apple deleted photos and videos stored in the new iCloud Photo Library, but allowed users to retain files on their iOS 8 devices. Content was automatically uploaded to Photo Library after the process was complete. the same automation should be available for iCloud Drive documents, though developers had to re-enable the feature using Apple's Setup Assistant.
Finally, email attachments sent with the Mail Drop feature will expire, meaning users have to resend messages with new attachment links. With Mail Drop, Apple bypasses the usual content attachment data limit by uploading to iCloud files up to 5GB in size, then includes a link in the message body for recipient access.
When Apple launches the anticipated operating systems this fall, both will feature deep integration with iCloud features like Dropbox-style storage in iCloud Drive and content sharing services like iCloud Photo Library.
Update: Apple has sent out a note to developers regarding the upcoming data wipe, clarifying that iCloud Drive, iCloud Photo Library and Mail Drop will not be affected by the procedure.
We will perform a server-side data wipe on all CloudKit public databases on July 22. If you are using iCloud containers with public databases, consider creating a copy of your data so that you can re-upload once the data wipe is complete. iCloud Drive, iCloud Photo Library, and Mail Drop will not be impacted by the targeted data wipe.