I believe a wave of invites to opt-ins (random people) went out Thursday night/Friday this week. Hopefully we can expect ours next Thursday night/Friday.
On a related note, I did download the client from:
http://www.wotlkwiki.info/index.php/Main_Page
...and am now installing the many patches that, naturally, are required afterward. Like most of you, I don't have my beta key yet, but I am able to get up-to-date now, so I recommend that you do so. This way, once I get my key, I can upgrade my account and immediately log in, rather than spend a full day downloading the patches.