Configuring NFS server on F8 not working

[Richard England]

I have a F7 (didius) and a F8 (poirot) machine that I am trying to 
"cross mount" directories on.

I can mount F7 directories on the F8 machine using   mount -t nfs 
didius:/home/foobar /mnt/foobar

But when I try the opposite, mounting the F8 directory on F7   mount -t 
nfs poirot:/home/rhombux /mnt/rhombux   it fails with the message

    "mount: mount to NFS server 'poirot' failed: System Error: No route 
to host."

I have tried with SELINUX and the firewall both disabled but that seems 
to have no effect.  The set up on each machine seems to be the same. 
I've set up /etc/exports on each machine

On the F7 machine (didius)
/home/foobar            poirot(rw,sync,no_root_squash)

On the F8 machine (poirot)
/home/rhombux        didius(rw,sync,no_root_squash)

exportfs -a returns no errors.  NFS seems to be enabled

[root at didius rengland]# /sbin/service nfs status
rpc.mountd (pid 2464) is running...
nfsd (pid 2461 2460 2459 2458 2457 2456 2455 2454) is running...
rpc.rquotad (pid 2446) is running...


[root at poirot rengland]# /sbin/service nfs status
rpc.mountd (pid 1990) is running...
nfsd (pid 1987 1986 1985 1984 1983 1982 1981 1980) is running...
rpc.rquotad (pid 1972) is running...

I can ping each machine from the other and I can use ssh to access each 
of the machines from the other.

Can anyone point me at some further diagnostics I can look at or perhaps 
give me some hints about other things I can try?  I had this working 
when both machines were F7.  I'm beginning to suspect and F8 issue but  
I'm at the end of my ideas.

I'm going to be away from the machine for a while but this is driving me 
further nuts and I had to ask.


Thanks,

~~Richard