@ewertz: At least on the Arduinos, you don't have to do any such modifications if you already have regulated 5V available elsewhere. All that you have to do is to connect your 5V *regulated* PSU to the POWER (the lower left-hand 6/8-pin) header's "5V" pin, and you're done.
Actually, I beg to differ. What you want is that if only the USB cable is plugged in you use that as a power source -- if only an external supply is plugged in you use that as the power source -- and if both the USB and an external supply are plugged in you use the external supply.
The problem with your suggestion -- driving 5V directly into the +5V pin on the header on the Arduino is that the VIN signal is on the input to the regulator -- the resistor divider would generate an outpout of 0V, so the comparator would enable the FET and allow the USB power to come through -- resulting in the USB and your external supply both trying to power the 5V rail.
This wouldn't be a mega issue if they were both at EXACTLY 5V all the time, but there are always going to be tiny differences between them, leaving them fighting with each other.
Max, I'm concerned about this circuit operating from USB power only. When the USB power is connected it will initially be fed to +5V through T1 since T1's gate will be low because there is no +5V to IC7 to pull it high. Once +5V charges up to near 5V, IC7 pin5 will become higher than pin 6 (3.3V) and T1 will turn off. Then +5V will start discharging (since it has no source of power). I suspect T1 will turn back on when +5V gets to around 4.5V. The circuit will probably oscillate and maintain +5V somewhere around 4.5V. This is enough to power the 3.3V regulator (I assume - I haven't read the datasheet), so the Arduino may run OK, but I would be concerned about anything (like shields) which are powered from +5V.
Have you measured +5V with a DMM or looked at it with an oscilloscope when powered ONLY by the USB power?
BY05V, If I understood your description correctly, connecting the external 5V to the 5V header pins would allow you to avoid the modifications to D1 and IC1 and power the Arduino from the external supply. However it does not eliminate Max's concern for having the USB power back-driving the external supply. You would also need to add a jumper from +5V to the resistor divider and add the 5.6K to make sure the USB power is disabled when the external supply is connected. I would also be concerned about the USB power back-driving IC1.
At least on the Arduinos, you don't have to do any such modifications if you already have regulated 5V available elsewhere. All that you have to do is to connect your 5V *regulated* PSU to the POWER (the lower left-hand 6/8-pin) header's "5V" pin, and you're done. This pin is on the 5V power "bus" that is either USB bus power or the output of the linear regulator, depending on which you've got connected.
So, basically, instead of using the 5V power pin as an output (for shields or jumpers to breadboards), you're using it as board power input.
You might look to see if this is also an option on the ChipKit boards. Unless there's something upstream in the power conversion/switching circuit that's going to get back-powered (to any detrement), this should also work.
BTW, I think that many of the 3V3-native boards *only* generate 5V as an intermediate power rail because there are so many legacy shields/circuits that still really do need 5V to work, so they still make it available as a convenience to the user. Otherwise, the 3V3 Arduino "compatibles" couldn't care less about the existence of 5V.