zoomkat:
I didn't look at the datasheet for the regulator being used now, but with something like a 7805 you're not supposed to allow the output pin to be higher than the input. A diode is often used to route voltage back to the input pin.
With a voltage drop across the diode, how would that work? The voltage regulator on my old arduino put the arduino +5v pin at 8v+ when externally powering using 12v in on the barrel jack. I then used a 7805 chip from the 12v to the +5v arduino pin and the arduino worked without issue.
Obviously the last diode drops worth of voltage won't be routed back, but will not cause a problem. If you measured +8V on the +5V pin with +12V connected to the barrel connector then something wasn't measured right. +8V would easily destroy the CPU.