http://www.fileplanet.com/promotions/bfbc/na/There's several other places giving them away as well but this is the one i used, follow instructions, register if you have to it's a bit of a lengthy one but you get 2 maps and one game mode to try out.
The previous i hear was crap on the xbox but this one looks promising.
It's also important to note this starts in FIVE days so these keys will go quick.
edit: Btw read the requirements, they have some odd things they expect you to have done before they give you it - like having to have done previous betas and played previous battlefield games, just lie about it i did.
