CrossRoads:
It would be connected to the OUTPUT of the onboard regulator but it wouldn't matter because the onboard regulator would have nothing connected to it's INPUT because it's INPUT is the Vin pin. It would essentially be like it wasn't there since no current would be flowing through it with nothing connected to the Vin.
The data sheet for the regulator recommends a diode from +5 (anode) to Vin (cathode) to avoid reverse driving the regulator like that. The Duemilanove had a more robust regulator that could take it.
Not the way I read it!
The NCP1117 family has two internal low impedance diode paths that normally do not require protection when
used in the typical regulator applications. The first path connects between Vout and Vin, and it can withstand a peak surge current of about 15 A. Normal cycling of Vin cannot generate a current surge of this magnitude. Only when Vin is shorted or crowbarred to ground and Cout is greater than 50 µF, it becomes possible for device damage to occur.
Strictly, the UNO has 47 µF capacitors, but the point is - you simply should not be shorting Vin to ground. It is true that the TI LM1117 specifies:
In the LM1117-N regulators, the internal diode between the output and input pins can withstand microsecond surge currents of 10A to 20A. With an extremely large output capacitor (?1000 ?F), and with input instantaneously shorted to ground, the regulator could be damaged.
It is clearly much more rugged.
You use a switchmode adapter which is regulated, and feed it directly as the 5V, no regulator needed on-board. Admittedly, almost none of the switchmode "wall-warts" include proper crowbar protection.
I think we are all agreed that not only would it be foolish to attempt to incorporate the (switchmode) power supply into a project unless you were undertaking a commercial product on such a scale as to undertake the CE, UL and other certification (very expensive process), but there simply is no way it would even be anywhere near as cheap.
Love that pricing for the "DHgate" adapter - US $2.38 - just as long as you buy a hundred. 