Google has updated everyone with details of what caused their recent faux pas that caused about 150,000 Gmail users ‒ or as Google would like to remind you 0.02% of their users ‒ to loose all their GMail data. This if nothing else should be a rude reminder to people that despite all the benefits of webmail, it's nice to have a nativeclient
Google would like to reassure their users that the data has not been lost, and they are working on a fix. Service is already beginning to resume to those affected by the issues, but the progress is bound to be slow as Google is restoring from backup tapes which can take quite a while.
According to Google a bug in an updated version of their storage software was responsible for the data loss in their servers affecting their redundant data stores. They have since reverted to an older version of the software.
If you are one of those affected by the issue, you can keep a check at Google's Apps Status Dashboard, which they will keep updated with even more details.