A good friend of mine officially has the best reason ever for not RSVPing: he didn't receive an invitation!
In our defense, we did send one. But he had just moved, hadn't sent out his new address, and hadn't updated his addy with USPS. And fortunately, he did receive our e-mailed save-the-date back in January and had already purchased plane tickets and a hotel room, so he won't miss our wedding because of it. But I think he was relieved to hear that he is, in fact, 100% officially invited!
So if any of you, like A.'s dad or Econo Boy, feel bad about following up with people, keep my friend T's story in mind. It may feel a bit awkward to ask, "so, uh, are you coming or what?," but it's nowhere near as awkward as a guest whose invitation went astray having to ask, "so, uh, am I invited?"