Unfortunately, there's a lot of secrets here -- given that they're selling a service, AT&T doesn't speak too frankly about how much downtime they're really responsible for, and of course as a trade secret they can't give any numbers on how much bandwidth is passing through and where it's all going. But they will say that they've got latency levels down to milliseconds (in their testing, I'm sure -- lots of players would probably suggest it's a little worse, depending on which ISP you use), and that they offer services like Synaptic Hosting. During times of hard usage, Blizzard can ask (for a price, of course) to open the floodgates up and make sure there's enough bandwidth to go around.
Fascinating stuff. It's too bad we can't get a really unbiased view of just how big Blizzard's operation is -- right now, with so much money in the system, everyone's got a financial interest to keep the details of how it all works to themselves. But maybe someday, when the game is shut down and Mike Morhaime writes his autobiography, we can get a nice wide look at all of the technology behind keeping Azeroth running.