Originally Posted by mattdm 
Hmmm. Does the flash do the wrong thing if you set ISO incorrectly?
I was amazed to find out upon testing that the flash and camera do the right thing and adjust the lens aperture if your camera ISO doesn't match the flash. Not only that but the flash adjusts automatically to the correct ISO when you turn the flash on so if you have ISO 400 set, the flash comes up with ISO 400 the first time. If you adjust ISO after that, the flash compensates with aperture.