Quote from WorldofWarcraft.com: (Source)
You read that right, Blizzard is looking for a "healthy handful of guilds" to join the closed beta testing of World of Warcraft: Cataclysm.If you think you and your guildmates have what it takes to make invaluable beta testers, take a stab at our Cataclysm Beta -- Guild Contest. All you need to do is tell us in a short essay why you and nine of your guildmates should be in the World of Warcraft: Cataclysm beta test. We’ll be picking 100 winning submissions each week for five weeks, for a total of 5,000 beta keys at stake!
What you need to do to is send email [email protected] a short letter between 50 and 250 words telling why they should pick you and nine of your guild mates for the beta.
Blizzard will be picking 100 winning submissions each week for five weeks (that's 1000 x 5 = 5000 people joining the closed beta). Picking of the submissions will start on Monday, June 7th and end on Monday, July 12th.
More information can be found here
Also: Don't forget to read the contest rules!