Keep in mind that only about 60 users lost emails in that incident. I believe the accounts were later restored from backup. And the cause of the data loss was outside of Google. I've never heard of Google itself losing any user data that was stored in gmail. Ever.
That said, there is a first for everything. Plus, very rarely, accounts can become temporarily unusable. (The data is safe, but temporarily inaccessible.) This usually fixes itself in a period of minutes to an hour. I would personally bet that the odds of your being unable to access your account because of a problem on Google are significantly lower than the odds of your being unable to access the internet because of a problem with your ISP. But if you are concerned, then by all means you can use IMAP to download all of your gmail data and back it up for yourself.
Note, I work at Google, but not on anything related to gmail. Nor do I personally know anyone on the gmail team. I only know about the incident above because I googled "gmail data loss" and it was the only story that turned up. I know something about how we backup data, but I'm not at liberty to discuss it. However I will say that I personally have great trust for Google's backup procedures.
I suspect that they don't back up per se but instead replicate data across multiple data centers.
Here's an interesting post about AppEngine which I expect applies to gmail. gdocs etc...
http://googleappengine.blogspot.com/2009/09/migration-to-bet...