Checking system logs on the Mac revealed nothing.
The Linux NFS server (Red Hat Enterprise 5) complained with this warning:
nfsd: request from insecure port (192.168.7.130:49232)!
After some Internet sleuthing, I found Mac client side NFS tries to mount NFS volumes over high TCP ports (>1024). You must explicitly tell the Linux NFS server to accept mount requests on high ports by adding the "insecure" option to /etc/exports. For example,
Then, NFS mounts from OS X should work as expected.