I managed to get one of the free keys via a link in the main BFBC2 thread, where they gave out a few thousand (VERY quickly), but other than those, I believe you had to preorder the game to get access.
ya some of the websites you bought the game from, are still giving out 2 beta keys, but idk which sites they were or if they were scams just be careful when looking around to buy the game, if i can come across 1 i will let you know.