mount.nfs: Unknown error 521

[Geoffrey Leach <geoff-1srLxb0qcfWsTnJN9+BGXg@public.gmane.org>]

I've got a simple network, systems 'A' and 'B' connected via wireless. 
B wants to mount A's /, /home and /usr/local. The fstab entries on B 
are:

A:/               /A                nfs	defaults	0 0
A:/home           /A-home           nfs	defaults	0 0
A:/usr/local      /A-ul             nfs	defaults	0 0

With this, B should do the mounts at boot time, assuming A is on line, 
which it is. (A:/home and A:/usr/local) are in seperate partitions)

The curious thing is that the mount of A:/ works fine. mounts of A:/
home and A:/usr/local fail with "mount.nfs: Unknown error 521"

FWIW, B is running Fedora 10 nfs-utils-1.1.4-8.fc10.i386, 
while A is running up-to-date Fedora 12, nfs-utils-1.2.1-4.fc12.i686.
This worked find prior to reloading F12 on A. 

Turns out that the problem was transient, but I'd like to know what was 
going on, if that's possible.

Thanks.