Since iOS 2.x the iPhone operating system comes with the capability to take screen shots from the current running application. This feature sometimes can be very useful, but the gesture to take the screen shots is very simple but it isn't very publicized.
To take the screen shot simply press the home and the sleep button at the same time, then go to Photos and then in the Camera Roll. In the Camera roll you'll find the taken screen shots.
This tip works fine on my iPhone3gs but it should work fine with other iPhones and iPod Touch, too.