All of a sudden, my NetBSD 6.1.2 SPARC NFS client failed to mount my FreeNAS share with this error:
mount_nfs: rpcbind to nfs on server: RPC: Program not registered
The only thing that recently changed was I upgraded my FreeNAS server from 8.3 to 9.2. Since several of my other NFS clients were able to connect to the share, I was doubtful the upgrade was the problem.
After a lot of digging, the problem turned out that I now needed to mount the share with the TCP option. So adding tcp as an option to my /etc/fstab fixed the problem.
nas:/mnt/volume1/backup /backup nfs tcp,rw 0 0
Based on this, I can only conclude some default changed in FreeNAS that forced me to have to add the tcp option to my NetBSD box.