This trick using a P channel mosfet is a common way to provide reverse polarity protection on a power supply input. It avoids the voltage drop problem of using a diode, with just the resistance of the mosfet to consider.
Switchmode DC-DC Converter Design Notes
The following diagrams are taken from:
http://www.analog.com/library/analogdialogue/archives/41-06/ground_bounce.html (more…)
