some useful reference about Kerberos Authentication

- How to: Enable Kerberos Authentication on a SQL Server Failover Cluster

http://msdn.microsoft.com/en-us/library/ms189585%28SQL.90%29.aspx

- ISA Server 2006 does not support using cross-domain or cross-forest trusts to include more than one Active Directory domain in a Kerberos constrained delegation scenario.

http://technet.microsoft.com/en-us/library/bb794858.aspx