The documentation for the [url=http://wiki.nginx.org/NginxHttpProxyModule#proxy_cache_path]proxy_cache_path[/url] directive of the [url=http://wiki.nginx.org/NginxHttpProxyModule]NginxHttpProxyModule[/url] states that the "Key and filename in cache is md5 of proxied URL". Here is an example:

a user requests:
http://nginx.foo.com/bar.html

nginx has the proxy_pass directive:
proxy_pass http://apache.foo.com/

the cached filename will be the md5 of:
http://apache.foo.com/bar.html

This worked for me when using the default proxy_cache_key with nginx 0.7.62