#4 - March 22, 2011, 4:35 p.m.
Unless you're actively seeking a beta invite through various promotions, contests, etc. or have attended BlizzCon, then your only chance is going to be getting an invite through the Battle.net beta opt-in.
If selected through the beta opt-in there's no key involved, and beta access is automagically added to your Battle.net account. You'll receive an email informing you of that.
The SC2 beta FAQ has quite a bit of info that's still helpful for the beta signup process:
http://us.starcraft2.com/beta-faq.xml
It's also important to be aware of beta related scams. If there's any email that claims you've been selected by the opt-in, don't click on any of the links, simply go to www.battle.net, log in, and you can see if you have been selected. Otherwise any promotions shouldn't require more than an email reply, and would include a beta key directly in the email (and again you can go directly to www.battle.net to claim it).