Though this may not have been the case earlier this morning, CM Nethaera has just confirmed that invites to the Wrath beta are indeed going out via email. So if you're opted in to the beta, check your email! I know I will be checking it many, many times today. If you don't get in, never fear; as the NDA has been lifted, we will keep everyone fully informed of all the latest news right here on the Insider.
If you do get in, you'll be asked to create a temporary account using your email and a new password. So be very, very wary of any email that asks you for your current username and password, since this is almost certainly a scam. Also, verify that any links you click are located at worldofwarcraft.com (and not, say, wow-blizzard.net). The scams are only going to come faster now that the beta is really going on, so be careful.
Edit: I should have remembered to mention this. Beta invite is completely random from the pool of people who have opted in.