Be wary of fake Battlefield 3 beta keys, says this Joystiq story. According to Battlefield 3 community site BF3blog, that website BF3nation is offering some fake Battlefield 3 beta keys to visitors. The site's owners were apparently received in a previous phishing scheme, so anything they offer is probably bullshit..
Joystiq says that EA has been trying to shut down the site for a while, but has so far been unsuccessful.
Right now EA isn't offering invites to beta keys for Battlefield 3, so anyone offering them is engaging in some sort of scam that will cause you nothing but trouble. Be vigilant. When Ea does start offering them you'll know because just about every game-related site on the Internet will be offering a chance to get one.
For now, assume anything related to BF3 beta keys is a swindle.