I got same problem...

2012-04-06 15:01:51,627 INFO    : Calling hook: preflight_check
2012-04-06 15:01:51,642 INFO    : Calling hook: set_defaults
2012-04-06 15:01:51,644 INFO    : Calling hook: bootstrap
2012-04-06 15:10:17,814 INFO    : Calling hook: configure_os
2012-04-06 15:10:29,295 INFO    : 
2012-04-06 15:10:29,296 INFO    : Current default time zone: 'Etc/UTC'
2012-04-06 15:10:29,300 INFO    : Local time is now:      Fri Apr  6 20:10:29 UTC 2012.
2012-04-06 15:10:29,301 INFO    : Universal Time is now:  Fri Apr  6 20:10:29 UTC 2012.
2012-04-06 15:10:29,302 INFO    : 
Extracting templates from packages: 100%
2012-04-06 15:11:14,237 INFO    : 
2012-04-06 15:11:14,238 INFO    : Current default time zone: 'Etc/UTC'
2012-04-06 15:11:14,243 INFO    : Local time is now:      Fri Apr  6 20:11:14 UTC 2012.
2012-04-06 15:11:14,248 INFO    : Universal Time is now:  Fri Apr  6 20:11:14 UTC 2012.
2012-04-06 15:11:14,248 INFO    : Run 'dpkg-reconfigure tzdata' if you wish to change it.
2012-04-06 15:11:14,249 INFO    : 
2012-04-06 15:12:15,274 INFO    : stop: Unknown instance: 
2012-04-06 15:12:39,350 INFO    : Updating certificates in /etc/ssl/certs... WARNING: Skipping duplicate certificate brasil.gov.br.pem
2012-04-06 15:12:39,484 INFO    : 0 added, 1 removed; done.
2012-04-06 15:12:39,500 INFO    : Running hooks in /etc/ca-certificates/update.d....done.
2012-04-06 15:12:48,388 INFO    : restart: Unknown instance: 
2012-04-06 15:12:49,261 INFO    : start: Unknown parameter: JOB
2012-04-06 15:12:51,128 INFO    : umount: /tmp/tmp206_J9/dev: device is busy.
2012-04-06 15:12:51,129 INFO    :         (In some cases useful info about processes that use
2012-04-06 15:12:51,129 INFO    :          the device is found by lsof(8) or fuser(1))
2012-04-06 15:12:51,130 INFO    : Cleaning up
2012-04-06 15:12:51,130 ERROR   : Process (['umount', '/tmp/tmp206_J9/dev']) returned 1. stdout: , stderr: umount: /tmp/tmp206_J9/dev: device is busy.
        (In some cases useful info about processes that use
         the device is found by lsof(8) or fuser(1))

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 118, in unmount_dev
    run_cmd('umount', '%s/dev' % self.context.chroot_dir)
  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 (['umount', '/tmp/tmp206_J9/dev']) returned 1. stdout: , stderr: umount: /tmp/tmp206_J9/dev: device is busy.
        (In some cases useful info about processes that use
         the device is found by lsof(8) or fuser(1))