Here's a question (I think I know the answer, but asking in public documents...):
I have in many cases built devices that use external power of one sort or another, some of them 5v, but other voltages (especially 12v) end up being switched by the Arduino to do useful things. In these cases, I have simply added a 5v regulator (7805 with a heatsink) to feed the Arduino board and connected the produced 5v directly to the 5v pin on the Arduino, with a common ground of course. Though I don't always do it, I also sometimes will add an electrolytic capacitor (something halfway reasonable, 100uf or more) after the regulator to smooth out the power a little bit. I figure it can't hurt when the supply power might be seeing jumps due to motors or whatever is being driven, and may be unregulated power in many cases.. in thinking about it, it would probably not be a bad idea to add another before the 7805. Caps are cheap, clean power is worth it. When I start running into problems with odd random behaviour and resets, power noise is a common reason the ATMEGA decides to flip out.
I would think that in some ways this is a superior power solution than using the onboard regulator, but the nagging question remains in my mind as to whether the traces to the leads on the board are okay with this method of providing power, and if there is any issues I hadn't otherwise considered?
Really great writeup, concise and a quick read. Going to check out some of his other posts, I always enjoy a well-written article like that..