I got in through opt-in. I was never given a key; it was already tied to my battle.net account.
I've been signed up for the opt-in since it was available almost two years ago (I think), but I haven't gotten a key yet. :(
if you go to your battle.net account management there's a link that says "Beta Opt-In" If you click it you can select what types of games you want to beta test, and then you submit your system specs, and it adds you to the pool of potential beta testers.
I have to brag. I got my invite code in the e-mail on Wednesday this last week. Actually, I saw I was chosen when I logged into my b.net account and saw a new game in among my list. I'm so sorry, but I love it. Multi-player only, but that's okay. It's still fun as hell.SC2 FTW !