I found these two articles very informative, In many of my project the user log in through a form and are validated against AD. From there i would create a NetworkCredential that i would pass to the sql reporting webservice. This is another method you can use to provide sql reports with out having to hit add.
Using Forms Authentication in Reporting Services:
Using SQL Reporting Services 2005 and Forms Authentication with the Whidbey/2.0 SQLMembershipProvider