MrGarak:
My tests so far with a 12v supply have led to no problems with the vreg even getting warm. It must have been the LCD screen's current draw causing the heatup on a previous project where I attempted to use a 12V supply.
Some LCD backlights need as much as 100mA, and drawing that amount of current from the 5V pin would certainly make the regulator hot.
MrGarak:
Anyway, I noticed when the power supply is disconnected and the board is only powered through usb, there is still ~50mv backfed through the barrel jack. I'm afraid if the pin switches the fet in this state, the solenoid will attempt to draw a lot of current from the 5v usb supply back through the barrel jack, which might burn something out in the board. Do I need to take this into account, or is there some kind of limiting in the arduino board which will prevent this?
Don'r worry about it. There is a diode in series with the barrel jack, and your high-resistance meter is registering the leakage current through the diode, which is probably well under a microamp.