On the NFS client side it manifests as directories being displayed as empty. When I then try to list the same directory locally on the server, both ls and nfsd lock up and cannot even be killed -9. I can use the PEFS mounted filesystem without problems over the network when I serve it from userspace e.g. with samba.
FreeBSD 8.2 with the zpool v28 patches on server. Ubuntu client.
I've just had the same problem between FreeBSD 10.2 and OpenELEC (Linux). A workaround that worked for me is NFS mounting with the "nordirplus" form the Linux side.