Check to make sure nfs daemon is running: /etc/init.d/nfs-kernel-server start - check the logs for any errors
I'd be checking '/etc/exports' for:
...
## BEGIN : DisklessMDRoots
/usr/pluto/diskless/46 192.168.80.0/255.255.255.0(rw,no_root_squash,no_all_squash,sync,no_subtree_check)
...
If you need to make changes try 'exportfs -a -v' before trying to boot your MD's
Check the file '/etc/fstab' on each orbiter: 'cat /usr/pluto/diskless/<MD#>/etc/fstab'
Look for something similar to the following:
...
192.168.80.1:/usr/pluto/diskless/46 / nfs intr,nolock,udp,rsize=32768,wsize=32768,retrans=10,timeo=50 1 1
...
Tell us how you went?
Cheers