The Windows Server System team has a blogwith lots of good information about Server 2003 R2 and Longhorn Server. One recent post talks about Unix and Server 2003 R2 interoperability. Quoting from that post:

"One of the many cool technologies in R2 is the Unix/Windows interop capabilities, such as authenticating users across platforms and file sharing across multiple OSes."

Authentication between different operating systems has been a royal pain for a long time and if this helps it's worth checking out!