Simultaneously press the Home and Power buttons (i.e. – press and hold the home button and briefly press the power button whilst the home button is held). The screen will flash white to indicate the screen capture has been done and you will find the screen shot in your camera roll in the Photos section, ready to be downloaded into iPhoto, etc When iPhoto loads the camera roll ready for importing, you’ll notice the screen shots do not have an associated thumbnail, just an blank empty thumbnail. Clicking the blank thumbs and hitting import selected still works though and they appear fine as PNG’s in the main Library.

Home screen right now:

My page for Apps:

My Web shortcuts page:

You’re not restricted to screen grabbing just the home pages either, you can do a screen shot whenever you want in ANY application, so you could screen grab your high score in SuperMonkey Ball and email the screen grab to your friends instantly, here’s a few examples:

NOTE – Do not press and hold the Home button and Power button for more than a few seconds, hold them too long and you will end up hard-rebooting your iPhone. No damage done or data lost but you need to know these things.