If you're using Basic authentication the username is available via HttpContext.Current.User.Identity
and the password is in some server variable (Enable Tracing to check which
one). Use this info to set the Credentials property on your webservice proxy
using ther NetworkCredentials class.
If you're using any other authentication mechanism you've got a problem since
you don't have a password. It's the classic 2nd hop authentication issue.
You need to somehow acquire the password from the user. The reason basic
authentication works is because the password is sent as a header to the server
when the user is challenged in the browser.