Cool, thank you vasquo! Good to know I'm not going insane!yes my beef is the way the diode is drawn and hence ALSO the drain and source terminals.In their schematic they have drawn the circuit symbol for a P MOSFET such that the Drain terminal of the mosftet is connected to the USB 5V when in fact they should have drawn it such that the source terminal is connected to USB 5V. Basically the MOSFET symbol is drawn back to front in their diagram.I thought it was just a simple error in the schematic drawing, however I was doubtful given this error has been present since the Duemilanove schematics back in 2009.... I can't believe this hasn't been picked up on to date?
I also struggled with this and thought D and S should have been swapped in the schematic (and on the board). But there is a good, simple explanation here of why it is correct as drawn:http://www.engineeredentropy.com/2013/01/arduino-power-supply-selector/#more-214In summary, the MOSFET is actually being used as a diode, not so much as a switch. But when the gate is low the channel conducts and the diode forward drop is essentially eliminated.If you swapped D and S and put it in a more conventional PMOS switch arrangement then the body diode will allow flow from the Arduino Vreg back to USBVCC when the gate is high and the channel is blocked. Jim