I saw that option - it's nice. I just wish I could liberate my gmail data too. Almost half a million emails, and I'd like to have a gzip of them in case something happened.
I use getmail to backup my Gmail (per that link), but there is still data loss. You can either use POP and lose your labels or use IMAP and lose your unread status (IMAP mode marks everything as read - someone patched getmail to offer a "leave unread" option but the maintainer refuses to accept any such patches).
Also, Google throttles your downloads - on POP this takes the form of returning just a few emails per download. It took me about three months to have a full backup at home, and I'm a paying customer (~30gb of mail on Apps Premier or whatever they're calling it today).
If this "read mail" thing is a problem for you why don't you just patch the software? Yes, a .tar.bz dl link in GMail would probably be nice, but sometimes you have to take matters into your own hands. If the maintainer doesn't want the patch, patch it yourself and host it on Github/Bitbucket/Google Code/etc.
I know, I just think Gmail should have a built in interface. It'd also be nice to be able to only download emails with a certain label, in a specific period of time, etc.
