There's been talk here and, really, all over the net about backing up Gmail locally -- largely because a few people have fallen prey to what seems to be spontaneous combustion of Gmail accounts. While some of these may very well be the result of violations of terms of service, I know I for one lost my account without ever violating any ToS and was given neither warning nor later justification for my loss off account.
That said, I personally feel anyone who uses gmail and values the information they keep on their account should take care to keep a backup of said data. It is a bit ironic, if not counter-productive, to backup locally something that urges you to "archive everything" and enjoy universal remote access -- unfortunately, all that convenience and archival is meaningless if you don't have an account. Unfortunately, sometimes gmail accounts just die.
Gmail does provide POP3 access to your account which, if nothing else, services are a rather efficient means of backing up your data.
Just go to your settings, select the POP/Forwarding tab, and make sure you select "All Mail" (even already downloaded) -- this will release all your mail for download to a third party client so you can download away. Just follow the setup instructions provided for your mail client of choice.
The real reason I started this thread is the following, tiny bit of information--because it threw me off at first. Gmail doesn't release all of your mail immediately. It releases blocks for 200 - 400 messages at a time. So when you go to download and it says it's downloading 300 or so messages, don't be alarmed. Next time you check your mail, it'll grab another block -- and so on. If you have a big gmail account, your best bet is to set your e-mail client to download every minute or two and just leave it running overnight. Eventually, it'll download them all. From there on out, it's easy street -- just downloading your daily mail traffic.
Any seasoned computer user knows the value of backing up data (even if they don't actively pratice it -- gmail is no different.