Hm, it would help if the placed "Page not found" and the 404-code more prominently. If I came across this on a random website, I think I'd have a hard time realizing that this is supposed to be a 404 error page.
A 404 should render as a 404 when javascript is turned off. I understand the site itself might require javascript but the 404 should be sacred bare bones.
Huh, that doesn't work for me. I tried both Chrome, Firefox, and Safari on OS X. I don't see any content from notfound.org (ie, their little iframe) in the source of the 404 page at all.
The actual 404 pages look like: http://notfound-static.fwebservices.be/404/index.html