Battlefield: Bad Company 2 MP beta starts in 2 weeks on January 25th and will run for a month until February 25th.
If you want to participate in the beta, right now the only way to do so is to preorder the game from GameStop or EA and receive your beta key. There should be other ways to get the key, but no word on that yet.
More on that here
So...... YOU ALL NEED TO GO PREORDER THE GAME! I KNOW I AM!