Single Signon for Mix Windows/Linux Domain

I’m setting up a new remote data center. It will initially have five servers — one Windows 2003 Server, and four Linux servers, probably SuSE.

I want all logins on the servers or local or remote workstations to be validated by a single machine. In a Windows-only domain, it would all be done through a domain controller and Active Directory. In a mixed environment, what’s the best way to go?

Thanks!
John