Bashiok posted a recommendation of how to make certain you get a Diablo III beta invite. While he did not necessarily confirm a beta announcement, he still replies several threads concerning beta.
It seems Blizzard Entertainment is moving into a different direction about providing beta access. Blizzard will no longer email customers a beta key, or provide links to get a beta key. From now on, Blizzard will flag your account to have access to future beta download pages. You will find out if you were randomly selected to participate in beta if your account contains access to the Diablo III beta page.
This is obviously a measure against the waves of scams and fake Blizzard emails which lure players to click links to fake login pages to steal your username and password.
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).