I'm working on some little projects that'll involve more than one Arduino (right now two of them, an Uno R2 and a Mega R3) along with some other 5V items that may themselves draw a few amps.
Rather than have multiple external power supplies, I'd like to have one 5V supply for the whole project, feeding all the Arduinos (which won't directly draw that much power) and the high-power part as well. (Alternatively, I would be willing to create a 12V->5V regulated supply just for the Arduinos - the base supply has a few buses to work with - since the high-power 5V line may sag a bit under its load (a long LED strip)... or just feed 12V to the wall adapter inputs of the Arduinos instead.)
All that said, if you have a nicely regulated 5VDC supply to work with already, how best to feed that to a few Arduinos? What happens if you then plug one of the powered Arduinos into a computer? Will this direct power supply and the USB power be in conflict?
(Note: My understanding from this related topic is that if you power an Arduino via the external adapter AND plug the USB into a computer, the external adapter still does all the powering. However, if you are feeding 5VDC regulated to the Arduino some other way (e.g. Vin) then I'm not sure how that'll work... assuming that's not Just A Bad Idea.)