Receptional_Andy - 12:40 pm on Mar 28, 2008 (gmt 0)

Your rewrite rule is just displaying the content of the 404 page, but it's not returning the correct (404) status code. That means broken links are essentially valid pages and will, for instance, get indexed by search engines.

You could probably replicate this, including status code with a rewriterule, but I doubt you would ever need or want to since errordocuments are designed for this purpose.