from Blizzard:

Today we opened the Wrath of the Lich King Beta Opt-In program, which gives players a chance to win an invitation to the upcoming Wrath of the Lich King beta. For The Burning Crusade beta, we randomly invited players from the entire World of Warcraft community. With the Opt-In, our goal is to strengthen the beta-testing process by inviting only those players who have expressed active interest in participating.

Keep in mind that opting in doesn't guarantee you'll be selected for the beta. And players who've previously been promised beta access through a promotion or contest (such as by purchasing the Dell XPS M1730 World of Warcraft Edition notebook) don't need to worry -- you'll still be entered into the beta automatically when the time comes.

To opt in to the Wrath of the Lich King beta, visit the Beta Opt-In page.