mod_auth_remote allows you to authenticate users on one (or more) remote web 
server(s), using basic HTTP authentication.
When a request comes in, mod_auth_remote obtains the client username &
password from the client via basic authentication scheme.
It then builds a HTTP header with authorization header built from the client's
userid:passwd. mod_auth_remote then makes a HEAD request to the authentication
server. On reciept of a 2XX response, the client is validated; for all other
responses the client is not validated.

WWW: http://puggy.symonds.net/~srp/stuff/