With the advent of the new iPhone firmware 2.0, users who have installed it report that you are now able to easily take and share screenshots of your iPhone -- all without downloading any additional software. To take a screenshot, just hold down the home button while quickly pressing the sleep/wake button at the top of the phone. The screen will flash and an image will be saved in your Camera Roll.
You can easily share your screenshot by e-mailing it using the share option in the Photos app.
For more iPhone tips and tricks like this, check out the iPhone 101 section here on TUAW.