Needless to say its simple and easy to retrieve a url before its been rewritten using asp.net Request object .This short article explains how to retrieve a url after it has been rewritten using url rewriting technique(isapi in my paticular case ).
Out of the many server variables available to our disposal the Request.ServerVariables("HTTP_X_REWRITE_URL") gives you the rewritten url.
A sample code line would be as follows in vb.net
Dim myUrl As String = Request.ServerVariables("HTTP_X_REWRITE_URL")
That should be all you need to do .Hope that helps