[some sniping]
> 
> But I am unable (re-)mount root or any other filesystem writable.  The
> machine hosting this is a Linux 2.0 box, with NFS 2.2beta29.  I've stopped
> and restarted mountd and nfsd, added the appropriate (I think) entries to
> /etc/exports: 
> 
> /usr/export/root/xhost     xhost(rw,no_root_squash)

> The mount command returns, no complaints or anything.  Is there something
> else I should do? How can I see (other than writing to it) if it is
> writable?  The 'mount' command under Linux provides this... 
> 
> I also have a DECstation 3100 (aka PMAX) which disk-boots NetBSD 1.3.2 and
> is able to NFS mount things writable from the Linux box, so there doesn't
> seem to be anything inherently wrong with NFS implementation.

   Three things:  Do you have an ident server running on the NetBSD/Decstation?
Also, are you running an internal name server?  If so, do you have a PTR record
set up properly for 'xhost'?  Finally, see what happens when you stick in a
FQDN in /etc/exports such as: 

/path/to/whereever   xhost.whatever.org(rw,no_root_squash)

    Do you have any /var/log/messages output from the linux server?   Set
debugging on in nfsd and see what happens when you attempt to mount the 3/60's
root filesysem.  I take it you don't have a small disk handy to boot a local
root filesystem with...  

    Try some of these things and mail back some SYSLOG output from the server 
if it continues to break.  You should be able to track this down if you turn 
on debugging - see man nfsd.

I love linux, but it does have the worst NFS implementation save for PCNFS.
Hope I helped some!
--jmg