FWIW, I thought I would pass this along since it was a little bit hard to follow the Dreamhost instructions, so it had to be tweaked for WP, but was VERY simple.
Create your custom pages.
For Dreamhost and WP, they must be named as follows:
401 /failed_auth.php
403 /forbidden.php
404 /missing.php
500 /internal_error.php
In the Dreamhost instructions, it says to name them as .html, but it seems to like it better if they are named as PHP.
Upload those files to your root directory.
Then, in the Templates Section on WP, add the following to your .htaccess file:
ErrorDocument 401 /failed_auth.php
ErrorDocument 403 /forbidden.php
ErrorDocument 404 /missing.php
ErrorDocument 500 /internal_error.php
That should do it.
My .htaccess file looks like this, the additional code you see with rewrite cond disallows hotlinking. Hotlinking is BAD, very BAD.
Options -Indexes
RewriteEngine on
RewriteCond %{HTTP_REFERER} !^$
RewriteCond %{HTTP_REFERER} !^http://(www\.)?tuckle.net/.*$ [NC]
RewriteRule \.(gif|jpg)$ - [F]
ErrorDocument 401 /failed_auth.php
ErrorDocument 403 /forbidden.php
ErrorDocument 404 /missing.php
ErrorDocument 500 /internal_error.php
To see my error pages in action?
http://www.tuckle.net/delete will give you a 403
http://www.tuckle.net/dogs will give you the 404.