I had been meaning to do this for so long and never did because I preferred Gmail over any native app that was available.
Luckily I tried Sparrow (http://sparrowmailapp.com) about 2 months ago and never looked back. I think the initial download of my "All Mail" folder took 12+ hours but I can rest assure I have a backup of my email on my hard drive and my backup hard drive (for which SuperDuper is awesome: http://www.shirt-pocket.com/SuperDuper).
A while ago I setup the Apple Mail app (forgot whether it was via pop3 or whatnot) to download new gmail messages, which more or less works as a way of replicating your gmail. But I often simply forget to run Mail, since I never have any other reason to use it. This reminds me that I should come up with a more robust solution.
You dawg I heard you like relying on the cloud for backup, so I backed up your cloud data on the cloud so you can rely on the cloud while you rely on the cloud.
Your comment is amusing, but there's merit to diversifying your cloud storage. I.e. if I lose access to gmail, I still have my dropbox stuff. Storing offline is another way to diversify, but it's subject to a lot of other vulnerabilities.
I think Greplin could provide this service fairly easily (since they are already indexing all of your data from other services for you and perhaps have access to your DropBox account.) Just using Greplin today as-is they could be a good cloud-based "backup" of a lot of your cloud-based data, so long as they do not automatically wipe out older data to match what is on the individual services.
Another service that could be really helpful for this problem would be one that would just automatically burn your DropBox account (which ideally had copies of your other cloud-based data) onto DVDs and send them to some location at some interval for automatic cloud-based offline backup. Not sure if you can automate the mailing part, though, and I would worry a little about ensuring that people get the right data.
A side point: is it a gmail "nightmare" or a cloud nightmare? the case in question doesn't describe a gmail specific vulnerability yet the alarmist headline hints otherwise. Backing up cloud data might a good hygiene but I call foul on the link-bail headline.
http://offlineimap.org/