Today there was a report of some data loss for 60 gmail users. They lost all their mail, their address books, etc. Lame.
This prompted me to look into methods of backing up my gmail account. Looks like it's a simple as changing a gmail setting (enabling POP3) and setting up a POP3 account in your favorite mailer (Thunderbird, Outlook, ...). It's a straightforward procedure. It takes a while to download all your mail the first time, and thereafter it just downloads new messages. It probably wouldn't allow a user to restore a mangled gmail account, but it would at least provide an external backup of all the messages. Unfortunately, it doesn't look like this process preserves message labels.
And it looks like you can export your address book, too: click 'contacts' (left-hand panel) and then click 'export' in the upper-right.