Does your invitation state that you need an RSVP for planning purposes? That might be the source of the confusion. I'm not a MySpacer, so I don't know how it works there, but I've used eVite for stuff (including our wedding, no joke) and it has a very clear response section... so does Facebook's "Events" system... it's "Attending," "Not attending," or "Maybe," which is at least more info than you're getting right now. You might want to just try re-sending the invite with a specific request for info, because people tend to be stupid.
