Would I be correct in assuming that a PNP transistor with a higher current capacity would draw more current when not in use?
This is the circuit that use P-MOSFET to switch the supply linehttp://www.edn.com/file/24877-Figure_2.pdfhttp://www.edn.com/design/analog/4362161/Transistor-latch-improves-on-off-circuitry
What is the role of the DC/DC converter?Also is the value of Q1 critial, or will any p channel mosfet be ok?
Hmm, would a bistable (latching) relay be a good idea? A button activates it, a circuit is closed and the power to the Arduino starts flowing. Arduino does its thing and when it's done it turns itself off by pulling its own plug.