#12 - Feb. 25, 2011, 6:05 p.m.
Right, so the most consistent method is going to be to opt-in through Battle.net.
There's actually still a guide available for StarCraft II, and much of the info is still accurate for the process to opt-in for the Diablo III beta.
http://us.starcraft2.com/beta-faq.xml
Aside from the normal opt-in there will undoubtedly be contests and promotions, such as the keys given away at BlizzCon. As we have info about additional ways you could potentially get a beta key we'll of course let you know.