just checked with a default installation of wordpress - and indeed it seems to be a default behaviour of wordpress to look for the path anyways - no matter what directories you add hide. So also replacing the "/2010/01/some-long-url" by "/error/some-long-url" will redirect the reuest to "/2010/01/some-long-url".
Anyways - I can still suggest two workarounds:
1) if you really want to get a real 404 error you can use the redirect method in your htaccess to forward the request to a non-existing url - just add the one redirect-line like this:
Redirect /podcasts /podcasts-error
all together the htacces could then look like this:
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
Redirect /podcasts /podcasts-error
</IfModule>
2) If you don't really need the 404 error you may also just add a page and give it the exact same url - in this case it would be "podcasts". That would de-activate the forwarding. You could then add your own custom 'error message' to that page..
Greetz, t..