OK, we had some pictures of early AM3+ socket, and there was no way you could fit BD into Am3 socket:
http://www.freeimagehosting.net/uploads/32f02b7d8c.jpg
you can see there is an extra hole in AM3+ socket, left hand side, left lower corner.
Of course maybe BD chips dont need that extra hole and AMD just cut it just for kicks. But where is the fun in that?