I'm sure the auditing and control is more than enough to stop your average creeper employee from reading normal people's inboxes, but I very much doubt that it's enough to stop a very smart, very determined spy from doing the same. At the end of the day, someone has root, and that guy can do pretty much anything.

I'm confident that Google is doing a better job than pretty much anyone else, but this problem is a more or less unsolvable one.

Edited to add that another interesting idea is that the people who man the DC's are actually pretty sparse (relatively few people for a lot of servers) so it's not inconceivable that one could trigger a failure on an important box, take down a replica of the figure's mailbox, swap out the drive for RMA and then do a quick copy. I bet this would be easy.

I guess my point is that no level of internal controls at any company can actually stop a determined government. If that were true, governments, which are much more paranoid than tech companies, would have eradicated spying a long time ago.