Here's what happens:
rzimmerman@hydra:~$ u1sdtool --start
Traceback (most recent call last):
File "/usr/bin/u1sdtool", line 39, in <module>
bus = dbus.SessionBus(mainloop=loop)
File "/usr/lib/python2.7/dist-packages/dbus/", line 211, in __new__
File "/usr/lib/python2.7/dist-packages/dbus/", line 100, in __new__
bus = BusConnection.__new__(subclass, bus_type, mainloop=mainloop)
File "/usr/lib/python2.7/dist-packages/dbus/", line 122, in __new__
bus = cls._new_for_bus(address_or_type, mainloop=mainloop)
dbus.exceptions.DBusException: org.freedesktop.DBus.Error.NoServer: Failed to connect to socket /tmp/dbus-QN3U2Wb4O0: Connection refused

It's client version 4.2.0 and Ubuntu 13.04.

dbus is running. However, I get a similar error if I do this:
rzimmerman@hydra:~$ dbus-monitor
Failed to open connection to session bus: Failed to connect to socket /tmp/dbus-QN3U2Wb4O0: Connection refused

sudo dbus-monitor does work though... huh?
Now, oddly if I run sudo u1sdtool --start, I get the following:
rzimmerman@hydra:~$ sudo u1sdtool --start
Oops, an error occurred:
org.freedesktop.DBus.Error.NoReply: Did not receive a reply. Possible causes include: the remote application did not send a reply, the message bus security policy blocked the reply, the reply timeout expired, or the network connection was broken.

So what's happening here? Seems like something odd is going on with dbus, but I don't know enough to be sure.