There are many customer who are still using Service for Unix ( SFU 3.5) in their environment and looking forward for support from Microsoft on issues which they are facing. Windows 2003 and SFU 3.5 are already out of mainstream support.
http://support.microsoft.com/lifecycle/?p1=3207 ==> Lifecycle support for SFU 3.5
Below link talks about the lifecycle support for Windows 2003:
SFU 3.5 installed over Windows 2003 R2 is unsupported and not recommended. Starting with Windows Server 2003 R2, NFS services were integrated into the operating system.