You must have iPhone 2.0 firmware installed to follow this tutorial.
To take a screenshot simply hold down the Home button and press the Sleep/Wake button. You will notice the screen flash white.
To view the screenshot you just took, press the Photos icon on your SpringBoard.
Press to select Camera Roll from the list of Photo Albums.
Press to select the small thumbnail of your screenshot.
Now you can see your high quality iPhone screenshot. You can easily sync this to your computer just as you would with any other photo!