I can report that the nfsd port to cygwin does work rather nicely with !Sunfish. The only problem I've had is the server dying now and then but it can be restarted via the services manager thingy in windows.
I had the server die when copying files so I used remote desktop, restarted the nfsd service and hit 'retry' on the risc os copy dialog. seems fine.
I've also heard it said that the Windows Services for Unix NFS server doesn't work reliably with !Sunfish. Anyone care to add to this?