Since mongrel and thin appear as the people favorites application server (with all my respects to Passenger), the configuration of webservers changed in order that you have a web server plus a kind of proxy and then a lot of application servers.

The idea is that, given a request, the webserver has to decide if it is an static or a dynamic url and then serve the request itself or pass it to the proxy of application servers..

The problem is that, as thin and mongrel are also web servers, if you don't take care with the configuration your application server can be serving all the static files too and be busy when it has to attend a dynamic request, with all the problems it causes.

That's why for me is very surprising when you search in Google about Apache + Mongrel, for example, and you arrive to virtual hosts configurations that doesn't take this into account.

The rules are very easy:

 RewriteCond %{DOCUMENT_ROOT}/%{REQUEST_FILENAME} !-f
 RewriteCond %{REQUEST_URI} !/stylesheets/.* [NC]
 RewriteCond %{REQUEST_URI} !/images/.* [NC]
 RewriteCond %{REQUEST_URI} !/javascripts/.* [NC]
 RewriteCond %{REQUEST_URI} !.*\.js$ [NC]
 RewriteCond %{REQUEST_URI} !.*\.jpg$ [NC]
 RewriteCond %{REQUEST_URI} !.*\.gif$ [NC]
 RewriteCond %{REQUEST_URI} !.*\.css$ [NC]
 RewriteCond %{REQUEST_URI} !.*\.js$ [NC]
 RewriteCond %{REQUEST_URI} !.*\.png$ [NC]
 RewriteCond %{REQUEST_URI} !.*\.ico$ [NC]
 RewriteCond %{REQUEST_URI} !.*\.bmp$ [NC]
 RewriteRule ^/(.*)$ balancer://mongrel_cluster%{REQUEST_URI} [P,QSA,L]

And this is an example: in order to your needs you can tune this configuration including more filet ypes and static directories.

For nginx the rule is also very simple.

So, you know, don't forge to include rewrite rules for serving static files.