virtualbox - Disable caching when serving static files with Nginx (for development)

We are using Nginx to serve static files on a development platform. As it is a development platform, we'd like to disable caching so that each change is propagated to the server. The configuration of the VHost is quite simple:

server {
  server_name  static.server.local;
  root /var/www/static;

  ## Default location
  location / {
    access_log        off;
    expires           0;
    add_header        Cache-Control private;

  } 
}

When we access an HTML file (http://static.server.local/test.html), we have no issue: the server returns a code 304 Not Modified as long as the file is not changed, and a 200 OK response with the modified file when the file is changed.
However, it seems to behave differently with a Javascript or a CSS file. Once the file is changed, we get a 200 OK response as expected, but with the old text.
Is there an internal cache mechanism in Nginx that could explain this behaviour? Or some configuration that we should add?

As a side note, here is the header returned by Nginx when the file has been modified (it seems correct):

Accept-Ranges:bytes
Cache-Control:max-age=0

private
Connection:keep-alive
Content-Length:309
Content-Type:text/css
Date:Fri, 13 May 2011 14:13:13 GMT
Expires:Fri, 13 May 2011 14:13:13 GMT
Last-Modified:Fri, 13 May 2011 14:13:05 GMT
Server:nginx/0.8.54

Edit
After trying different settings with the expires directive and Cache-Controlheader, I made some further investigations. In fact, the server is installed on a VirtualBox guest Ubuntu, and data are read from a shared folder that is on the Mac OSX host.
If the file is edited from an IDE (NetBeans) on the host, it seems that changes do not appear whereas if I edit it directly on the guest (using VIM), it is refreshed.
The strange thing is it does not behave similarly with HTML files.
Quite puzzling.

Edit 2 (ANSWER)
Indeed, the origin of the issue was more on the VirtualBox side. Or rather a conflict between VirtualBox and the "sendfile" option of the server.
This link VirtualBox Hates Sendfile gave me the solution: switch the sendfile flag in the server configuration to off:

sendfile  off;

Hope this could also help other person using VirtualBox for development. :)
There are some additional information on the VirtualBox forum.