[Update: As expected, Gmail's Web interface came back up roughly 100 minutes after going down. To check Google service status in the future, see the Apps Status Dashboard. Google has also posted a note explaining what went wrong. -Adam]
I'm still working on a set of articles about why I've transitioned from Eudora to Gmail, and not just Gmail, but Gmail's Web interface as viewed through the lens of Mailplane. Since that switchover, I've been a bit of an evangelist for Gmail's rethinking of how email should work, but having Gmail go down today was a distinct downer, not that I really needed to use email at the moment.
I fully expect that Gmail will come back up quickly - it has every other time it has gone down. But I do worry a little that there could at some point be a catastrophic failure at Google that would result in the loss of my email. That's why I use another email client (Apple Mail at the moment) to connect to my Gmail account via IMAP and bring all messages down to my Mac (POP would work too, but then I'd need to do local filtering). I almost never view my email via Apple Mail, but it's there in case I need it.
So I was intrigued to discover that while Gmail's Web interface was reporting a Server Error and telling me to try again in 30 seconds, Mail had no problem connecting to Gmail via IMAP and pulling in the latest messages.
The moral of the story - backup, backup, backup. The smart computer user who can't necessarily afford to be offline for a long period of time backs up not just data, but access methods. If Gmail's Web interface goes down, I can use IMAP. If Gmail stops working entirely, I can use my own server or my MobileMe account. If my cable modem goes down, I know where I'll take my MacBook for Internet access. And so on...