If using consolekit 0.2.1 with gdm 2.19.4 (in my case, consolekit 0.2.1-1 from Debian sid recompiled on Ubuntu gutsy and gdm 2.19.4-0ubuntu3), I observed the following behaviour:

To reproduce:
 1. Start up and log in as user A
 2. Request to switch to user B and log in (this will start a gdm flexiserver for B)
 3. Request to switch to user A, so as to return to A's session
 4. In A's session, log out.  The system will correctly switch to B (prompting for B's password)
 5. In B's session, log out.  Now the system fails to switch back to the gdm static login screen where A's session was running.

In some installations it is necessary to do this:
 1. Log in as A
 2. Switch to B
 3. Switch to C
 4. Switch to A and log out
 5. Switch to B and log out
 6. Switch to C and log out

This is because the static server doesn't count as a session for consolekit, the static server is not guaranteed to always be on the same VT, and X servers normally tend to return to the VT they were invoked on, when they are shut down.

The fix is to have consolekit know which VT contains the main static gdm greeter.  Then if it can't find a session to make current, it can switch to the greeter.

The patches I will shortly attach to this bug report implement this behaviour with a new dbus message SetX11ParkingPlace.