Hurry and visit your account management page now! You can opt-in for a chance to get into the Wrath of the Lich King beta like me! Now, you are not guaranteed a spot, but you might as well sign up, right?
Nobody knows when beta will actually start, but it sure seems like it'll be soon. My guess is late July/early August.
I was never in the original WoW or Burning Crusade beta, but I hope that I get into this one!