It seems that on the RFu328 at least, pins 11 & 12 on the Arduino float high when there is no power to the device. This is with a 1M pull-down resistor to ground. Probably something to do with them being usable for FTDI programming.
Which makes them no use to drive the MOSFET for the soft power switch as it's then 'always on'.
Moving the power connection to pin 2 fixes this. I'm using pin 11 to drive a little vibration motor but if that spins briefly on boot that's no big deal, so I've left that in use.