You can find this anywhere, but I am putting this entry up for my own reference:

To shoot a screenshot of any screen on the iPhone, hold down the Home and Sleep buttons simultaneously until the screen flashes. If you hold them down TOO long the phone will try to shut itself off.

It will create a PNG image of the screen and save it to the camera roll. I've found it works best if you hit the Home button slightly before the Sleep button. The PNG may take a minute or so to show up.