Most recent kernel where this bug did not occur:
vanilla 2.6.18
all 2.6.18-mm[1..3] have the bug.

I did a git-bisect to track it down to this commit:

51b6ded4d9a94a61035deba1d8f51a54e3a3dd86 is first bad commit
commit 51b6ded4d9a94a61035deba1d8f51a54e3a3dd86
Author: Trond Myklebust <Trond.Myklebust@netapp.com>
Date:   Fri Sep 15 16:31:56 2006 -0400

    NFSv4: When mounting with a port=0 argument, substitute port=2049

    RFC3530 states that the registered port 2049 for the NFS protocol should be
    the default configuration in order to allow clients not to use the RPC
    binding protocols.
    If the mount program sends us a port=0, we therefore substitute port=2049.

    Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

:040000 040000 f093abbb7cb6a01155ee2d2a820839ca2e9edadb 
4f90adecb2095bc921bcac83d9cff3c0fc1f5388 M  fs

Distribution: Gentoo-2006.1
Hardware Environment: x86_64 server, i386 client
Software Environment:
NFS-Server: tested with 2.6.18-mm2, but the same error could be seen with 
2.6.17-rc6-mm2 as server.
NFS-Client: 2.6.17 and 2.6.18 work, but 2.6.19-rc1 fails with the same 
configuration

The fstab entry of the failing mount:
192.168.0.4:/portage /usr/portage nfs4 rw,noatime,intr,noauto 0 0

Problem Description:
When trying to mount this share the mount process hangs several minutes, then 
it will display: "mount: 192.168.0.4:/portage: can't read superblock"
The syslog will contain: "nfs: server 192.168.0.4 not responding, timed out"

There is no firewall between the two systems, and the server does listen on 
port 2049.
rpcinfo -p localhost on the server:
   program vers proto   port
    100000    2   tcp    111  portmapper
    100000    2   udp    111  portmapper
    100024    1   udp  32769  status
    100024    1   tcp  51226  status
    100021    1   udp  32770  nlockmgr
    100021    3   udp  32770  nlockmgr
    100021    4   udp  32770  nlockmgr
    100003    2   udp   2049  nfs
    100003    3   udp   2049  nfs
    100003    4   udp   2049  nfs
    100021    1   tcp  39360  nlockmgr
    100021    3   tcp  39360  nlockmgr
    100021    4   tcp  39360  nlockmgr
    100003    2   tcp   2049  nfs
    100003    3   tcp   2049  nfs
    100003    4   tcp   2049  nfs
    100005    1   udp    739  mountd
    100005    1   tcp    742  mountd
    100005    2   udp    739  mountd
    100005    2   tcp    742  mountd
    100005    3   udp    739  mountd
    100005    3   tcp    742  mountd

netstat --listen --inet contains these lines:
tcp        0      0 0.0.0.0:2049            0.0.0.0:*               LISTEN
udp        0      0 0.0.0.0:2049            0.0.0.0:*

Steps to reproduce:
Export NFS:
/var/export 192.168.0.2(fsid=0,rw,no_root_squash,sync)
/var/export/portage 192.168.0.2(rw,nohide,no_root_squash,sync)
Try to mount it via above fstab-line:
mount /usr/portage