Having done some with ISA Server 2006 before in pubslishing SharePoint sites and Exchange services (OWA, Active Sync, HTTP-RPC), i thought it would great to utilize ISA's capability in carrying out authentication for servers.
Here is an article which talks about something similar.
The final solution had a MOSS doing Windows Integrated and SQL Server proivder based FBA on MOSS. ISA was used to provide single URL access to MOSS.
It would be nice if FORM based authentication in ISA can be customized to support additional providers like SQL Server etc.