It should be noted that in Debian 4.0 (Etch) the portmap utility may not install properly (I think there is something in the configure portion of the package that is broken).  This may in turn cause long delays for mounts to work as well as break any file locking operations over NFS.  The workaround to this is on both client and server system run:

 dpkg-reconfigure portmap

then make sure to say "no" at the 'bind portmap to localhost' question

Also, you may need to add entries on both servers in /etc/hosts.allow:

client system: 

    portmap: 192.168.0.100

    statd: 192.168.0.100

server system:

    portmap: 192.168.0.101

    statd: 192.168.0.101

Note that later versions of nfs-kernel-server will gripe at each filesystem export about a missing option in the exports file no_subtree_check | subtree_check.  I advise going with 'no_subtree_check' as an option added to each export.  This will of course vary with the operations you expect to be performed on the NFS share.