Mark's been doing a lot of work on his site lately, and one of the things he did was create a custom error page that allows people to use shortcuts in the url bar to access his content. He based this on the functionality of the PHP site, which allows you to search for the documentation of a function by appending it to the url. A while ago, I saw Razmus Lerdorf give a presentation about how they accomplished this on the php site. You can see his slides here. The part about the error pages starts around slide 44.