Thanks for the prompt feedback. I'm pretty sure the Arduino is hogging the power, since the sensor's listed power supply is 8mA, and I'm drawing roughly 100mA (burning through a 45ah 12V in two weeks), I think I calculated that right...
So I think I'll try the Arduino Pro Mini -- can anyone recommend between the 3.3V vs. 5V? I think the x-band sensor works with either, but I'm not sure what the functional difference would be. [http://www.parallax.com/Store/Sensors/ObjectDetection/tabid/176/ProductID/606/List/0/Default.aspx?SortField=ProductName,ProductName].
I read through the linked tutorial on cutting power consumption. I'm having trouble understanding what exactly it means to put the board to sleep. Is it possible to have the motion sensor actively sensing for motion, but also have the board in some sort of power saving mode?
Thanks again for the feedback, I appreciate the help.