While I was at BlizzCon, I met someone who worked at Blizzard in their IT department, and the stories he told me (all off the record, unfortunately) about how gigantic and powerful their systems are were just mindboggling. I had to ask if he knew of anyone or any other industry shooting information around at the scale that Blizzard does, and with the one possible exception of Google (who also run digital information around at a staggering rate), we couldn't think of anyone doing things at this scale. Not even financial networks and health care information (things, you'd think, that would be much more important than your level 73 Hunter and his gear) have these kinds of systems running.
Very, very interesting. I'd imagine that much of what Blizzard does consists of secrets they'd like to keep (hence the reason the Blizzard employee asked me to keep much of what he told me off the record), so it's not likely we'll be able to know all that much about how they do what they do (until it's all turned off, anyway -- maybe someday we'll see a tell-all detailing how WoW was played). But man -- everything I've heard about how the game is run involves unbelievable amounts of information running inside lots and lots of servers and wires. The architecture of what Blizzard has done in terms of size and scale rivals anything Arthas has constructed in Icecrown.