Well, to be fair, I can think of no full AT&T failure, but there have been dozens of Blackberry outages. There were two back in December that had long downtimes, one in March, and then I stopped paying attention.
While as you say, it's obvious RIM doesn't have one server running the whole show, that doesn't change the fact that RIM is a single point of failure for all Blackberries nationwide. If RIM goes down, so does your device. Not a good situation.
IIRC, if RIM goes down, it doesn't take down your entire phone. I believe BES and BBM is disabled, other than that, you're still able to make phone calls and send/receive texts.