After you've gotten the SQL Reporting Services Forms Authentication sample working, the next logical thing to do is access the Web Service itself while your SSRS instance is running under forms auth...but how do you authenticate?
Rather than populating the credential cache of the ReportServer instance with Windows credentials, you need to call the LogonUser method ala:
rs.LogonUser(formsAuthUserName, formsAuthPassword, null); However, there's still work to do. You also need to extend the proxy class of ReportingService on the client a bit so that it can handle forms-auth related cookies.
To get a great copy-and-paste example of how to do this, just hit books online and look up the LogonUser method...then check out the RSClient class that makes up part of the sample code.