I've tried searching online and can't find anything. Is there a specific component that will allow one PCB to be powered by both a USB and a wall wart? Not simultaneously, of course. Will a transistor suffice?

 

An OR gate with a driver?

 

Wouldn't an OR gate allow both simultaneously? I don't think that's what he wants.

An XOR might be better.

 

There are IC's designed just for that thing, switching between either battery/wall wart power or USB check here for more info on all available devices from Maxim-IC >>http://www.maxim-ic.com/products/hot_swap_power_switching/

The plus side is, you can get samples!
B. Morse

 

Wouldn't an OR gate allow both simultaneously? I don't think that's what he wants.

An XOR might be better.

You are right here, mistyped! Sorry