I couldn't agree with you more, Nathan. That is one of my pet peeves also. There are times when I want to simply lock the thread when the OP never comes back to respond or even say a simple "Thanks", but I don't because we get enough flack when we close threads that really need closing.
Actions such as you described are akin to someone walking up to a group of people, asking a question, and then walking away as the question is being answered. They never look back or respond to those answering the question they posed. I'm sure it's not really like that since members can view their threads without having to ever respond in it again, but the appearance of ignoring those who are trying to help is very real.
[soapbox mode off]