We heard a little while back that it was AT&T who provide data center hosting to Blizzard and this gigantic game (and actually, we've had outage problems before due to maintenance on AT&T's end), but our friend Tamara Chuang of the Orange County Register went straight to the source, and spoke with the big bandwidth provider on just what it takes to keep the servers up. There's some good information in there, especially if you're interested in all of the motherboards and wires that run the World of Warcraft. MMOs are apparently AT&T's biggest gaming customers, and they run the wires for companies like Blizzard as well as Konami and Turbine. They originally helped run Battle.net, and when Blizzard wanted to expand with World of Warcraft, AT&T's gaming division expanded with them.
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.