Straight from the horses mouth (Ok, Nethaera's mouth anyway):

The Wrath of the Lich King invitations have started to go out, and we have officially begun the beta test. As with The Burning Crusade, the beta-test process for Wrath of the Lich King will occur in phases, during which we will be inviting players who have opted-in to participate. https://www.worldofwarcraft.com/account/beta-opt-in.html?ticket=ST-3401534-Hnk2gUxJas2J3XUVdYYa

Players who have been invited to participate in the beta test will need to create a temporary Blizzard Account by visiting here: https://beta.worldofwarcraft.com/account/creation/start.xml

For more information on the Wrath of the Lich King beta test, please read the FAQ. https://beta.worldofwarcraft.com/expansion/faq.html

As always, be suspect of those invite emails (I'm sure the scammers will kick into overtime today) but there is a chance it could be valid. Good Luck!

Also, the NDA for WoTLK has been lifted! MMO-Champion has a TON of new information that they are updating now that there is no NDA. I'm off to do a little research on the new Inscription and Runeforging.