I went ahead and transposed the source and drain. Finally, just recently, I sent it off to seeed studio to have another ten boards made, and now I expect it will work correctly.
I have another question about the same design. As you can see in the schematic I posted with the first question, I put a 51k resistor tied between the gate and the input voltage. I did that so as not to leave the gate floating during switching the switch, but since I have had more time to think about it, I realized I didn't do anything to guard against switch bounce. As is, the resistor will keep the gate voltage from bouncing when turning the switch OFF, but when turning it ON, every time the switch contacts hit, the gate will go low, and then it will be pulled high through the resistor every time the switch bounces and loses contact. As I mentioned, this feeds a 7805 and a LD1086-3.3, linear regulators. The board has a place for an 0805 capacitor at the input of each regulator, and I specified 0.33 uF in the design, so that will help somewhat with switch bounce, but I am not sure if I am comfortable with this "solution" to the switch bounce.
1st suggestion for a better solution: I could add a small circuit with a capacitor and Schottky-input inverter, to debounce the signal before applying it to the gate. I've been using the circuit at
http://www.labbookpages.co.uk/electronics/debounce.html in some other circuits I designed.
2nd suggestion - an alternative solution: Why don't I just leave the resistor out of the circuit, and let the gate float? For the short time that the switch is between positions, the capacitance of the MOSFET gate will hold it in whatever state it was in. Or, I could make the resistor a much larger value such that its resistance times the gate capacitance is longer than the expected switch bounce time. That way, it would still function to keep the circuit switched off in case of mechanical failure of the switch, but under normal conditions the gate should float and retain most of its charge for the amount of time the switch is between positions or bouncing.