Hey Serge, I can reproduce this and I'm using the same versions mentioned in the bug description.

I added the three lines you suggested to /usr/lib/python2.7/dist-packages/VMBuilder/plugins/ubuntu/dapper.py and this is what I got:

vmbuilder kvm ubuntu --suite=oneiric --flavour=virtual --arch=i386 -o --libvirt=qemu:///system --ip=192.168.0.181 --gw=192.168.0.1 --part=vmbuilder.partition --templates=mytemplates --user=alexis --name=Alexis --pass=changeme --addpkg=vim-nox --addpkg=unattended-upgrades --addpkg=acpid --firstboot=/var/lib/libvirt/images/vm1/boot.sh --mem=256 --hostname=vm1 --bridge=br0
2011-12-03 22:10:10,852 INFO    : Calling hook: preflight_check
2011-12-03 22:10:10,872 INFO    : Calling hook: set_defaults
2011-12-03 22:10:10,873 INFO    : Calling hook: bootstrap
2011-12-03 22:25:20,818 INFO    : Calling hook: configure_os
2011-12-03 22:31:23,887 INFO    : invoke-rc.d: policy-rc.d denied execution of start.
2011-12-03 22:31:25,977 INFO    : update-rc.d: warning: unattended-upgrades start runlevel arguments (2 3 4 5) do not match LSB Default-Start values (none)
2011-12-03 22:31:25,977 INFO    : update-rc.d: warning: unattended-upgrades stop runlevel arguments (0 1 6) do not match LSB Default-Stop values (0 6)
2011-12-03 22:31:25,990 INFO    : invoke-rc.d: policy-rc.d denied execution of start.
2011-12-03 22:31:30,825 INFO    : 
2011-12-03 22:31:30,829 INFO    : Current default time zone: 'Etc/UTC'
2011-12-03 22:31:30,829 INFO    : Local time is now:      Sun Dec  4 00:31:30 UTC 2011.
2011-12-03 22:31:30,829 INFO    : Universal Time is now:  Sun Dec  4 00:31:30 UTC 2011.
2011-12-03 22:31:30,829 INFO    : 
2011-12-03 22:31:49,387 INFO    : 
2011-12-03 22:31:49,388 INFO    : Current default time zone: 'Etc/UTC'
2011-12-03 22:31:49,390 INFO    : Local time is now:      Sun Dec  4 00:31:49 UTC 2011.
2011-12-03 22:31:49,390 INFO    : Universal Time is now:  Sun Dec  4 00:31:49 UTC 2011.
2011-12-03 22:31:49,390 INFO    : Run 'dpkg-reconfigure tzdata' if you wish to change it.
2011-12-03 22:31:49,391 INFO    : 
2011-12-03 22:31:58,682 INFO    : lsof: WARNING: can't stat() fuse.gvfs-fuse-daemon file system /home/alexis/.gvfs
2011-12-03 22:31:58,682 INFO    :       Output information may be incomplete.
2011-12-03 22:31:58,682 INFO    : lsof: status error on %s/dev: No such file or directory
2011-12-03 22:31:58,682 INFO    : lsof 4.81
2011-12-03 22:31:58,682 INFO    :  latest revision: ftp://lsof.itap.purdue.edu/pub/tools/unix/lsof/
2011-12-03 22:31:58,683 INFO    :  latest FAQ: ftp://lsof.itap.purdue.edu/pub/tools/unix/lsof/FAQ
2011-12-03 22:31:58,683 INFO    :  latest man page: ftp://lsof.itap.purdue.edu/pub/tools/unix/lsof/lsof_man
2011-12-03 22:31:58,683 INFO    :  usage: [-?abhlnNoOPRtUvVX] [+|-c c] [+|-d s] [+D D] [+|-f[gG]]
2011-12-03 22:31:58,683 INFO    :  [-F [f]] [-g [s]] [-i [i]] [+|-L [l]] [+m [m]] [+|-M] [-o [o]] [-p s]
2011-12-03 22:31:58,683 INFO    : [+|-r [t]] [-s [p:s]] [-S [t]] [-T [t]] [-u s] [+|-w] [-x [fl]] [--] [names]
2011-12-03 22:31:58,683 INFO    : Use the ``-h'' option to get more help information.
2011-12-03 22:31:58,683 INFO    : Cleaning up
2011-12-03 22:31:58,684 ERROR   : Process (['lsof', '%s/dev']) returned 1. stdout: , stderr: lsof: WARNING: can't stat() fuse.gvfs-fuse-daemon file system /home/alexis/.gvfs
      Output information may be incomplete.
lsof: status error on %s/dev: No such file or directory
lsof 4.81
 latest revision: ftp://lsof.itap.purdue.edu/pub/tools/unix/lsof/
 latest FAQ: ftp://lsof.itap.purdue.edu/pub/tools/unix/lsof/FAQ
 latest man page: ftp://lsof.itap.purdue.edu/pub/tools/unix/lsof/lsof_man
 usage: [-?abhlnNoOPRtUvVX] [+|-c c] [+|-d s] [+D D] [+|-f[gG]]
 [-F [f]] [-g [s]] [-i [i]] [+|-L [l]] [+m [m]] [+|-M] [-o [o]] [-p s]
[+|-r [t]] [-s [p:s]] [-S [t]] [-T [t]] [-u s] [+|-w] [-x [fl]] [--] [names]
Use the ``-h'' option to get more help information.

Traceback (most recent call last):
  File "/usr/bin/vmbuilder", line 24, in <module>
    cli.main()
  File "/usr/lib/python2.7/dist-packages/VMBuilder/contrib/cli.py", line 216, in main
    distro.build_chroot()
  File "/usr/lib/python2.7/dist-packages/VMBuilder/distro.py", line 84, in build_chroot
    self.call_hooks('configure_os')
  File "/usr/lib/python2.7/dist-packages/VMBuilder/distro.py", line 67, in call_hooks
    call_hooks(self, *args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/VMBuilder/util.py", line 165, in call_hooks
    getattr(context, func, log_no_such_method)(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/VMBuilder/plugins/ubuntu/distro.py", line 155, in configure_os
    self.suite.unmount_dev()
  File "/usr/lib/python2.7/dist-packages/VMBuilder/plugins/ubuntu/dapper.py", line 124, in unmount_dev
    run_cmd('lsof', '%s/dev')
  File "/usr/lib/python2.7/dist-packages/VMBuilder/util.py", line 120, in run_cmd
    raise VMBuilderException, "Process (%s) returned %d. stdout: %s, stderr: %s" % (args.__repr__(), status, mystdout.buf, mystderr.buf)
VMBuilder.exception.VMBuilderException: Process (['lsof', '%s/dev']) returned 1. stdout: , stderr: lsof: WARNING: can't stat() fuse.gvfs-fuse-daemon file system /home/alexis/.gvfs
      Output information may be incomplete.
lsof: status error on %s/dev: No such file or directory
lsof 4.81
 latest revision: ftp://lsof.itap.purdue.edu/pub/tools/unix/lsof/
 latest FAQ: ftp://lsof.itap.purdue.edu/pub/tools/unix/lsof/FAQ
 latest man page: ftp://lsof.itap.purdue.edu/pub/tools/unix/lsof/lsof_man
 usage: [-?abhlnNoOPRtUvVX] [+|-c c] [+|-d s] [+D D] [+|-f[gG]]
 [-F [f]] [-g [s]] [-i [i]] [+|-L [l]] [+m [m]] [+|-M] [-o [o]] [-p s]
[+|-r [t]] [-s [p:s]] [-S [t]] [-T [t]] [-u s] [+|-w] [-x [fl]] [--] [names]
Use the ``-h'' option to get more help information.