Several of the messages posted seemed to imply that creationists mostly understand when they've lost a debate, and I don't think that's they way they see it, so I thought I'd offer my own thoughts.
I think most here would agree that those least able to participate in constructive discussion draw their ranks primarily from the creationists. There's a good reason for this: those arguing the position of science are fighting on their home turf. For most creationists science is foreign and very unfamiliar territory. To use
archaeologist as an example, he seems to know very little about anything except how to keep a discussion from getting anywhere, often by arguing two contradictory positions while never seeming to notice. Click on the link and read his threads. For a more complex example, read the still active
The End of Evolution By Means of Natural Selection thread and see
Faith exhibit the same behavior.
What drives this behavior is a certainty that they're right, and so when they leave this site it isn't because they feel they've lost, but for other reasons, few of them good. They feel they've either been treated unfairly, or lied to, or ganged up on, or misrepresented, or misunderstood, or ridiculed, or not taken seriously, etc. None of these reasons a creationist might have for abandoning the debate do anything to advance the understanding and acceptance of evolution and science by the conservatively religious community.
So I think our challenge should be to increase the chances that a creationist might come away from EvC Forum with positive feelings. I don't think convincing them that evolution is true is as important as giving them an understanding that those who accept evolution are human beings honestly seeking meaning just like themselves.
--Percy