There is a wealth of information on 404 pages, if you would like to read more I've posted my 404 resources here.
Here is my up-to-date list of 404 resources, they are mostly IIS or inspiration sites. Basically it covers anything I could fit in when I was writing my 404 articles.
Error 404 Links
Duncan Morris talks about the things developers need to remember before they launch a site.
Making your website usable by your users should be the goal. 404 pages are part of the solution.
50 Creative and Inspiring 404 Pages brought to us by the Webdesigner Depot
I don't use IIS (EVER!) but if you do here is a good resource.