am-utils not mounting NFSv3 but NFSv2

Binary package hint: am-utils

Description: Ubuntu 10.04.1 LTS
Release: 10.04
am-utils:
  Installed: 6.1.5-12ubuntu2
  Candidate: 6.1.5-12ubuntu2

I would like to automount shares with NFSv3. It is mounting the shares with NFSv2.

The logs indicate the share is mounted using NFSv3 :
Mar 24 16:11:35 georges amd[27943]/info: call_mountd: NFS version 3, mount version 3
Mar 24 16:11:35 georges amd[27943]/info: recompute_portmap: NFS version 3 on nfsserver.mydomain.com
Mar 24 16:11:35 georges amd[27943]/info: Using MOUNT version: 3
Mar 24 16:11:35 georges amd[27943]/info: call_mountd: NFS version 3, mount version 3
Mar 24 16:11:35 georges amd[27943]/info: autofs: unmounting /net/nfsserver/s10 succeeded
Mar 24 16:11:35 georges amd[27943]/info: nfsserver.mydomain.com:/s10 unmounted fstype nfs from /net/nfsserver/s10

/etc/mtab shows that my share is mounted using NFSv3 :
nfsserver.mydomain.com:/s10 /net/nfsserver/s10 nfs rw,intr,grpid,acregmin=3,acregmax=60,acdirmin=30,acdirmax=60,vers=3,proto=tcp 0 0

But a 3GB file output an error :
ls -l /net/nfsserver/s10/moniez/myfile.dat
ls: cannot access /net/nfsserver/s10/moniez/myfile.dat: File too large

And cat /proc/mounts shows that, in fact, the share is mounted using NFSv2 :
nfsserver.mydomain.com:/s10 /net/nfsserver/s10 nfs rw,relatime,vers=2,rsize=8192,wsize=8192,namlen=255,acregmin=0,acregmax=0,acdirmin=0,acdirmax=0,hard,proto=udp,port=65535,timeo=7,retrans=3,sec=sys,mountport=65535,addr=111.222.333.444 0 0
(I masked the IP address for the report...)

I tried to add nfsv3 in the amdmap options and nfs_vers = 3 in amd.conf without any luck.

The same version of am-utils on Debian Lenny works fine.