Starts on March 25th....

Battlefield: Bad Company on FilePlanet - North America - Home

Seems pretty easy to get the keys unlike some other beta tests but I am sure they will be running out soon so act fast if you want in. Obviously you need a gold XBL account and even more obvious an Xbox 360

EDIT: From what I am hearing the free public keys are all gone so unless you have an IGN Insider or paid Fileplanet account you are out of luck for now. Always a chance it will be like the COD4 keys where they add some more later part way through the beta testing.