I had been correct in assuming that those who did not respond to our wedding invitations were not able to attend; either it was culturally acceptable to simply not respond with one's regrets, or, more often, those who did not reply were simply not reliable enough to recognize that we actually did need to know if they were coming to our wedding (for reasons that are, to me, obvious).
As Miss Manners recently replied in response to a reader who thought one only need respond to a wedding invitation if one were attending: "Silence is an insult, not a response...These people thought enough of you to invite you to their wedding. You don't have to go, and you don't have to send a present. But ignoring them - not replying and not sending your good wishes - is callous."
(Can you tell that it still irritates me when people don't have their acts together?)