Using an authenticated Forms user's credentials?

Hello,

 First time poster here, and I hope someone here can answer my question.

I am developing an ASP.NET site that is backed by a particular suite of web services.  Both the site and the web services reside on the same server, both use Forms authentication, and use the same Membership store coming from the same database.

The web services I am using implement permissions logic of their own, so I am trying to find a way to pass the credentials of the authenticated ASP.NET user to the web services when I access the web services from the ASP.NET code. The code does not have access to the users' unencrypted passwords, so it cannot create credentials that way.  Does anyone know how to do this?  Putting <identity impersonate="true"/> in web.config is not an option, as the ASP.NET site uses the network service account to access the database, but without that, CredentialCache.DefaultCredentials does not return t ...