To get single sign on in the Cloud Microsoft offers Windows Indentity Framework (WIF)  a new addition to the .NET Technologies Stack formerly known as Geneva Framework. WIF is now available as a Release Candidate from MSDN Eval Center together with it’s server part Active Directory Federation Service V2.0 which offers a secure Web-Service for AD as an end-point to Cloud Apps according to WS-Federation and SAML Protocols.

If you want single sign on in the cloud that is what you need