Well, I've said it before and I'll probably say it again, in fact, here I am saying it. Don't close the lid with the machine on. At least not for any extended amount of time. Heat is dissapated through the keyboard and wristrest. Not a major portion of it but still quite a bit. And with the lid closed that heat has nowhere to go and so it builds up. Where do you think it would go? Its not real good for the LCD panel either. Actually, the build up is greater than with the lid open and the machine cranking, because when its doing that, the processors heat up and the fans come on. But with the machine idling, the processors are just leaking a little heat that dissapates up through the keyboard because the fans aren't blowing. But once there, its got nowhere to go so the temperature rises. The processors are probably just fine down below.
What's the point of closing the lid with the machine on? Why do it? I don't like to leave my machine running solo. Its probably silly, nothing will happen, but then again, you never know. If you have to close the lid for some neurotic reason, at least leave it ajar, say a half inch.