There was a Diablo III open beta over the weekend. You may have played it. Then again, you may have sat around like a bunch of other people trying to play it and being unable to because Blizzard's servers were melting.
Which sucks, seeing as its essentially a singleplayer game, but hey, welcome to 2012.
Anyway, those wondering just what kind of capacity it would take to create server issues with a company like Blizzard, know that the number of people playing concurrently "peaked at around 300k".
Which is a lot. And that's what betas are for! So Blizzard can say, OK, 300,000 people created problems, so when the game actually launches, we need to be ready for more.