Keep in mind that this is an alpha announcement -- beta has not officially begun. No beta keys have been emailed out. If you receive an email offering a beta key, ignore it for now -- as more information about the new expansion releases, phishing attempts are like to rise as well. Check out Blizzard's full announcement regarding the Alpha, and keep your eyes peeled -- with a client available, datamining will likely begin soon.
Update: Ion "Watcher" Hazzikostas had the following to say about the Alpha:
Don't read too much into the word "Alpha." We have a LOT of content that's ready for testing. http://t.co/c93xZhSOt1- Watcher (@WatcherDev) April 3, 2014