Well just because someone has to say it...
who is for the subject of the sentence
and whom is for the the object.
Either way, you mix them up is wrong. Neither is any more wrong than the other of course.
But I think when people throw in an unnecessary whom, it's probably because they're trying to be a smarty-pants, which is bad enough in and of itself. Being a smarty-pants and getting it wrong though, that's just awful.