tim7:
Isn't it the same ATmega168 chip on both 3.3V and 5V boards? The chip spec says it can run at 1.8 to 5.5V, although it needs lower clock speeds at lower voltages. So if I apply 5V to the Vcc pin of a 3.3V Pro-Mini board, it should be ok?
I think so. The key thing is you can't apply a voltage to a input pin that is higher then the voltage powering the chip.
Lefty