**UPDATE**: This works for the iPad as well. A friend gave me a tip today about how to take a screenshot on any iPhone running firmware 2.0. Press the sleep and home buttons simultaneously. The screen will flash white, and a screenshot will be saved into your Camera Roll as a .png image.
To get the photos off of the iPhone, you can e-mail them out or import them into your computer as you would any other image in the Camera Roll. The screenshots have no thumbnails, but this isn't a big deal (and is a known bug).
In prior versions of the iPhone firmware, I tried to screenshot various issues that I wanted my friends to take a look at. Unfortunately, I had to jailbreak my iPhone in order to get screenshots working. I then discovered that they didn't work on the "Call Failed" screen, which pissed me off even more. The firmware 2.0 method is much easier. :)
screenshots don't have thumbnails, due to a bug
(the missing thumbnails are the screenshots)
Images taken by the iPhone's camera seem to have accurate thumbnails, but note the images of my friend's newborn, which are unusually stretched. These were saved from Safari on the iPhone (by tapping and holding down one's finger on the image).