I'm seeing the ~/Mail/inbox/.mh_sequences file in my NFS mounted homedir 
acquiring extra NUL chars on the end.

I can reproduce this simply, using two client machines.  One of them (warthog, 
say) uses echo to create a file:

        echo hello >~/hello

Then a second machine (hades) extends it:

        echo hello >>~/hello

Then the first machine looks at it again:

        od -c <~/hello
        0000000   h   e   l   l   o  \n  \0  \0  \0  \0  \0  \0
        0000014

Note that the file has been extended by as many NUL characters as were in the 
original write.  I suspect that it will cut off at a page boundary as NFS will 
be forced to go and fetch a new page at that point.

I've attached a wire trace of the first client (warthog) covering the echo/od 
sequence.  Note that though there's a write (packet 14) for the echo on 
warthog, there's no read to show that the data is being read back.  warthog 
knows the file is bigger - it learnt that in packet 21.