KE7GKP:
Many Arduino boards (Including Duemilanove and Uno) have automatic power switching.
Nothing like the official documentation for authoritative answers. Better than 100 opinions here.The Arduino Uno can be powered via the USB connection or with an external power supply. The power source is selected automatically.
http://arduino.cc/en/Main/ArduinoBoardUno
Yes, but that doesn't state which voltage source is select if both are avalible (external takes priority). And the OP had a setup where he is hard-wiring an external regulated +5vdc voltage source to the Arduino's +5vdc shield pin, so in his case there might be three different voltage sources and his question was relevant.
The Arduino automatic voltage switching circuit has added complexity from the prior manual jumper placement where one could 'force' voltage source from only USB or only external power source (or Vin pin) or neither, if one wished to wired an external regulated voltage source to the +5vdc pin.
What we are dealing with is the undesired, but workable in practice, of having two (or more?) active voltage sources 'hardwired' together, which is normally not a recommended good engineering practice from any voltage regulator datasheet I've read that talked about that issue.
Lefty