Making 404 doorway pages

What's your thoughts on this

[bufferzone]

Just resived this in an e-mail fron another list. What's your oppinion???

<--- start e-mail snip
I've seen a funny thing here lately. In the last few days, I've been getting
traffic from search engines to pages on my site that I removed a few weeks
ago.

Idea: create real pages for your website and remove them later after they are
indexed. Have it serve up a 404 page with a complete site map of your site.
Bamn, your whole site gets indexed. Just imagine what you could do if your
404 page is dynamically created.

Oh, a cool thing I've done over the last years is making my php scripts end
in .html and configure apache to run them.
<---end e-mail snip

[mdharrold]

My understanding is that if spiders get a 404, they quit.
If this is true, they won't search any of those links.

[physics]

Yeah, even with a custom 404 script, the spiders will still be alerted of the 404. But I don't know if spiders ignore those pages. There are about a million results on Google for the search
404 not found
The top ones involve tutorials of course, but if you look at page 5 for example you will see some real 404 pages. So it seems that if your script returns the page with the same name as the one that the person was looking for each time you may be able to get away with this (but if the SEs catch on, i.e. they see tons of similar, doorway looking pages you could be in big trouble). You should be able to implement something like this with mod_rewrite though. Anyone want to comment more specifically on how this might be done in such a way that the SE doesn't know it's a 404?