I also get the 'Someone stole the video device!' error as well. I've looked through the conf as much as I can but I can't see anything that seems helpful. When it works, it works just fine, and exactly as intended, but after you kill it, you have about a 1% chance of being able to start it back up again unless you reboot the tablet from the console.

Meanwhile, other camera apps continue to work perfectly fine, including the Nokia Camera application, Gizmo, and even Gstreamer can accept the camera just fine. But even after running those and going back to try to run Motion again, it still complains about Someone stole the video device.

Perhaps Motion needs some flushing or something when it shuts down to ensure that when it tries to start up, there aren't any temp files being left anywhere that are causing it.