We had one guest who came to only the reception (we had 3 seats for them), they showed up as two, came for reception, and left immediately after. 3 wasted seats. The worst part is that I wasn't really even expecting money or anything like that, but would have at least liked a card that said congratulations. We did so much for them in the past, that we were beyond pissed that they couldn't even acknowledge our wedding day. We've cut them out of our lives - I can't deal w/ crap like that. It was the biggest day of our lives, and they treated it like a trip to the mall or something.
We also had someone else not give us anything, but are still getting the 'it's coming' from them...at this point I could honestly care less, and just will keep it in the back of my mind when they have something important in their life. They won't get jack sh*t. I hate to have this thinking, but it's the way it has to be. It's not that hard to go get a card from CVS, and write something nice in it.