Is there a way to take a screenshot of an Android device and save it as an image file?

I've been told rooting is the be all and end all, but at the moment, I want to see how far I can extend my device without rooting it. One thing that would be good is screenshot functionality, but I can't seem to find an application on the marketplace that does this without a ** ROOTED PHONES ONLY ** disclaimer. Is there any way to get screenshot functionality without a rooted phone? And what is the reason for this functionality …