According to Deathknight.info, European beta invitations are being sent out at this very moment. This is great news for all of our friends overseas! Please be sure to check your email to see if you've received one.
The above photo is from Deathknight.info and is an example of an email they have obtained. It would stand that U.S. invites are soon to follow, and that will mean those lucky enough will get to explore new areas of Northrend very soon.
Click the image for a high res view.
More Wrath of the Lich King information as the night goes on...
Edit: I want to share an interesting fact a friend of mine found out during the wait for the U.S. Beta Opt-In to start a few weeks ago. He noticed that the Beta Opt-In accessibility started at 12:00 p.m. (noon) France time for the European servers. He then predicted that they would start in the U.S. at 12:00 p.m. Pacific time. He was correct. European beta invites appear to have started going out at 6:00 a.m. France time, and we can venture a guess as to when U.S. invites may begin: 6:00 a.m. Pacific time. Again, this is only a guess, but a good one.