jdMorgan - 10:12 pm on May 4, 2010 (gmt 0)
A 302-Found response for invalid URLs is unacceptable.
For "missing pages" there are three valid responses: A 410-Gone for intentionally removed resources, a 301-Moved Permanently redirect to a specific replacement page or corrected URL if one exists, or a 404-Not Found if neither of the previous conditions apply.
Returning the correct server status is more important than anything else, especially whether the error page is pretty or not. Search engines don't even look at the error page, they only look at the server status response code.
Returning a 302 status means that any URL requested from your server for which content cannot be provided will result in a 302-Found redirect, followed by a 200-OK if the redirected=to URL is subsequently requested. So the search engine view of this is that an infinite number of URLs exist on your site, and they will arbitrarily limit the depth to which they are willing to crawl your site.
If you watch your logs carefully, you will see occasional requests for "nonsense" URLs coming from Googlebot, Slurp, etc. -- These are not mistakes, nor are they accidental. They are testing your server to see if it returns proper error codes.
You might want to dig around in Joomla-specific forums to see if there is a better solution, or if someone makes a "plug-in" to fix this issue. If there is no solution, please post back here, because I'm sure that many webmasters considering Joomla would abandon the idea if they can't even get 404-Not Found responses right...
[added] To avoid MSIE's "we're smarter than Webmasters" error pages and to ease your testing, consider using Firefox and the "Live HTTP Headers" add-on for Firefox and other Mozilla-based browsers. Alternately, go into MSIE's advanced options and turn off "Show friendly HTTP error pages." [/added]