endoscient,
I have a 20" ACD and I just connect all my cables to my PB, close the lid, and wiggle my mouse. The external display then turns on with my dock, desktop wallpaper ect.
I don't think you have to click the spanning box because that's telling your PB that you want both screens to be used instead of the external display only.
Make sure that you have the AC adapter plugged in for your PB or the external will only span and mirror. I don't think there is anything else that could be wrong. What rev. is your PB? Maybe your video card cannot power your external display?