This seems to be a new development, (all the articles I've come across that mention it are recent) but apologies if this has been known for a while. Yeah though, preorder at a Gamestop, and you get a beta key. If you go to the store, it'll be on your receipt. If you order online, it'll be emailed to you. Note that Gamestop claims you can't cancel your pre-order once you've received your key (and this is probably the case online), but I've read several accounts of people being able to cancel at the store and retain their key. I have nothing but anecdotal evidence for that, but I figured it was worth mentioning.

Anyway, whoo! Not only can you play SC2 without resorting to the shadier parts of the 'net, but you can play against real people, too. Hard to beat that. If anyone goes after this offer and starts playing, we forum-dwellers would love to hear your take on the current state of the game (I assume the NDAs are gone).