While this might be iPhone 101 to some, Apple pretty much snuck this feature into their 2.0 firmware, and while we're now at 2.1, not everyone may yet know that they can capture their iPhone screen contents with just the touch of a button -- well, two buttons actually.
To take a screenshot with your iPhone or iPod Touch, just press the Home and Sleep buttons at the same time. On 2.0, the screen will flash white, while with 2.1, you'll also get the camera shutter sound, and your screen will be captured.
To access your screenshots go to Photos and on the iPhone, look in Camera Roll, while on the iPod Touch, check out Saved Photos. (see, poetically, screenshots after the jump!)