I can confirm this bug.  It's been bothering me for a while now.

In the following error log, I first ran 'bzr viz' and it failed, then I re-ran it again and it succeeded.

cdb@deagle:~/mls-work/mls-cdb-int$ bzr viz vsc/src/com/venture/vsc/network/HeadNodeConnected.java
/usr/lib/python2.6/dist-packages/bzrlib/plugins/gtk/ui.py:224: DeprecationWarning: bzrlib.progress.ProgressBarStack was deprecated in version 1.12.
  self._progress_bar_stack = progress.ProgressBarStack(klass=widget)
bzr: ERROR: dbus.exceptions.DBusException: org.freedesktop.DBus.Error.Spawn.ChildExited: Launch helper exited with unknown return code 0

Traceback (most recent call last):
  File "/usr/lib/python2.6/dist-packages/bzrlib/commands.py", line 727, in exception_to_return_code
    return the_callable(*args, **kwargs)
  File "/usr/lib/python2.6/dist-packages/bzrlib/commands.py", line 922, in run_bzr
    ret = run(*run_argv)
  File "/usr/lib/python2.6/dist-packages/bzrlib/commands.py", line 559, in run_argv_aliases
    return self.run(**all_cmd_args)
  File "/usr/lib/python2.6/dist-packages/bzrlib/plugins/gtk/commands.py", line 189, in run
    pp = start_viz_window(br, revids, limit)
  File "/usr/lib/python2.6/dist-packages/bzrlib/plugins/gtk/commands.py", line 157, in start_viz_window
    from bzrlib.plugins.gtk.viz import BranchWindow
  File "/usr/lib/python2.6/dist-packages/bzrlib/plugins/gtk/viz/__init__.py", line 14, in <module>
    from branchwin import BranchWindow
  File "/usr/lib/python2.6/dist-packages/bzrlib/plugins/gtk/viz/branchwin.py", line 18, in <module>
    from bzrlib.plugins.gtk.tags import AddTagDialog
  File "/usr/lib/python2.6/dist-packages/bzrlib/plugins/gtk/tags.py", line 27, in <module>
    from bzrlib.plugins.gtk.revisionview import RevisionView
  File "/usr/lib/python2.6/dist-packages/bzrlib/plugins/gtk/revisionview.py", line 33, in <module>
    from bzrlib.plugins.gtk import seahorse
  File "/usr/lib/python2.6/dist-packages/bzrlib/plugins/gtk/seahorse.py", line 33, in <module>
    crypto = dbus.Interface(bus.get_object(BUS_NAME, CRYPTO_PATH),
  File "/var/lib/python-support/python2.6/dbus/bus.py", line 244, in get_object
    follow_name_owner_changes=follow_name_owner_changes)
  File "/var/lib/python-support/python2.6/dbus/proxies.py", line 241, in __init__
    self._named_service = conn.activate_name_owner(bus_name)
  File "/var/lib/python-support/python2.6/dbus/bus.py", line 183, in activate_name_owner
    self.start_service_by_name(bus_name)
  File "/var/lib/python-support/python2.6/dbus/bus.py", line 281, in start_service_by_name
    'su', (bus_name, flags)))
  File "/var/lib/python-support/python2.6/dbus/connection.py", line 622, in call_blocking
    message, timeout)
DBusException: org.freedesktop.DBus.Error.Spawn.ChildExited: Launch helper exited with unknown return code 0

bzr 1.14.1 on python 2.6.2 (linux2)
arguments: ['/usr/bin/bzr', 'viz', 'vsc/src/com/venture/vsc/network/HeadNodeConnected.java']
encoding: 'UTF-8', fsenc: 'UTF-8', lang: 'en_US.UTF-8'
plugins:
  bzrtools             /usr/lib/python2.6/dist-packages/bzrlib/plugins/bzrtools [1.14]
  dbus                 /usr/lib/python2.6/dist-packages/bzrlib/plugins/dbus [unknown]
  gtk                  /usr/lib/python2.6/dist-packages/bzrlib/plugins/gtk [0.96.0.dev.1]
  launchpad            /usr/lib/python2.6/dist-packages/bzrlib/plugins/launchpad [unknown]
  netrc_credential_store /usr/lib/python2.6/dist-packages/bzrlib/plugins/netrc_credential_store [unknown]
  qbzr                 /usr/lib/python2.6/dist-packages/bzrlib/plugins/qbzr [0.9.8]
  rebase               /usr/lib/python2.6/dist-packages/bzrlib/plugins/rebase [0.4.4]
  svn                  /usr/lib/python2.6/dist-packages/bzrlib/plugins/svn [0.5.4]
*** Bazaar has encountered an internal error.
    Please report a bug at https://bugs.launchpad.net/bzr/+filebug
    including this traceback, and a description of what you
    were doing when the error occurred.
cdb@deagle:~/mls-work/mls-cdb-int$ bzr viz vsc/src/com/venture/vsc/network/HeadNodeConnected.java
/usr/lib/python2.6/dist-packages/bzrlib/plugins/gtk/ui.py:224: DeprecationWarning: bzrlib.progress.ProgressBarStack was deprecated in version 1.12.
  self._progress_bar_stack = progress.ProgressBarStack(klass=widget)




Here's the relevant portion of my ~/.bzr.log file:

Fri 2009-05-15 15:08:30 -0700
0.032  bzr arguments: [u'viz', u'vsc/src/com/venture/vsc/network/HeadNodeConnected.java']
0.042  looking for plugins in /home/cdb/.bazaar/plugins
0.043  looking for plugins in /usr/lib/python2.6/dist-packages/bzrlib/plugins
0.453  encoding stdout as sys.stdout encoding 'UTF-8'
0.837  Deprecated method called
Called from:
  File "/usr/lib/python2.6/dist-packages/bzrlib/plugins/gtk/__init__.py", line 74, in set_ui_factory
    bzrlib.ui.ui_factory = GtkUIFactory()
  File "/usr/lib/python2.6/dist-packages/bzrlib/plugins/gtk/ui.py", line 195, in __init__
    self.set_nested_progress_bar_widget(ProgressBarWindow)
  File "/usr/lib/python2.6/dist-packages/bzrlib/plugins/gtk/ui.py", line 224, in set_nested_progress_bar_widget
    self._progress_bar_stack = progress.ProgressBarStack(klass=widget)
  File "/usr/lib/python2.6/dist-packages/bzrlib/symbol_versioning.py", line 132, in decorated_method
    trace.mutter_callsite(4, "Deprecated method called")
1.488  Traceback (most recent call last):
  File "/usr/lib/python2.6/dist-packages/bzrlib/commands.py", line 727, in exception_to_return_code
    return the_callable(*args, **kwargs)
  File "/usr/lib/python2.6/dist-packages/bzrlib/commands.py", line 922, in run_bzr
    ret = run(*run_argv)
  File "/usr/lib/python2.6/dist-packages/bzrlib/commands.py", line 559, in run_argv_aliases
    return self.run(**all_cmd_args)
  File "/usr/lib/python2.6/dist-packages/bzrlib/plugins/gtk/commands.py", line 189, in run
    pp = start_viz_window(br, revids, limit)
  File "/usr/lib/python2.6/dist-packages/bzrlib/plugins/gtk/commands.py", line 157, in start_viz_window
    from bzrlib.plugins.gtk.viz import BranchWindow
  File "/usr/lib/python2.6/dist-packages/bzrlib/plugins/gtk/viz/__init__.py", line 14, in <module>
    from branchwin import BranchWindow
  File "/usr/lib/python2.6/dist-packages/bzrlib/plugins/gtk/viz/branchwin.py", line 18, in <module>
    from bzrlib.plugins.gtk.tags import AddTagDialog
  File "/usr/lib/python2.6/dist-packages/bzrlib/plugins/gtk/tags.py", line 27, in <module>
    from bzrlib.plugins.gtk.revisionview import RevisionView
  File "/usr/lib/python2.6/dist-packages/bzrlib/plugins/gtk/revisionview.py", line 33, in <module>
    from bzrlib.plugins.gtk import seahorse
  File "/usr/lib/python2.6/dist-packages/bzrlib/plugins/gtk/seahorse.py", line 33, in <module>
    crypto = dbus.Interface(bus.get_object(BUS_NAME, CRYPTO_PATH),
  File "/var/lib/python-support/python2.6/dbus/bus.py", line 244, in get_object
    follow_name_owner_changes=follow_name_owner_changes)
  File "/var/lib/python-support/python2.6/dbus/proxies.py", line 241, in __init__
    self._named_service = conn.activate_name_owner(bus_name)
  File "/var/lib/python-support/python2.6/dbus/bus.py", line 183, in activate_name_owner
    self.start_service_by_name(bus_name)
  File "/var/lib/python-support/python2.6/dbus/bus.py", line 281, in start_service_by_name
    'su', (bus_name, flags)))
  File "/var/lib/python-support/python2.6/dbus/connection.py", line 622, in call_blocking
    message, timeout)
DBusException: org.freedesktop.DBus.Error.Spawn.ChildExited: Launch helper exited with unknown return code 0

1.490  Traceback (most recent call last):
  File "/usr/lib/python2.6/dist-packages/bzrlib/plugin.py", line 407, in _get__version__
    version_string = _format_version_tuple(version_info)
  File "/usr/lib/python2.6/dist-packages/bzrlib/__init__.py", line 103, in _format_version_tuple
    raise ValueError("version_info %r not valid" % (version_info,))
ValueError: version_info (0, 96, 0, 'dev', 1) not valid

1.490  return code 4