Why does the iPhone 3.1 take photos with power and home botton?

Hi, there!

That isn't really taking a picture on the iphone -- it's taking a screenshot. Although it makes the shutter sound, the camera really has nothing to do with this function.

On any iphone, holding the power button and then pressing the Home button takes a picture of whatever's onscreen at that time. The picture is saved to your camera roll like any other photo.