afremont:
But that's precisely why you would use that pin, to bypass the linear regulators.
Several posters have reported damage to their on-board 5vdc regulator by applying a regulated +5vdc to the 5V pin. Arduino folks seemed to have changed the brand/module of regulator on the rev3 boards. CrossRoad member feels the regulator doesn't like having a external voltage applied to it's output with no input voltage on it's input. The regulator's datasheet suggests adding a diode across the regulator's input and output terminals to protect the regulator.
And that is where the topic has been for some time now. The Arduino folk's warning has not come with a technical description of the why for their warning.
Lefty