It's the little things that you learn that make life easier. I wanted to screenshot the RushCrowds iPhone app this morning, turns out is easy:

1. Open the screen you want to copy
2. Hold down the power and the home button at the same time for a second then release, the screen flashes white and you hear a camera click. (The power button, which might be officially called the sleep button I suspect, is the one at the top right, the home button is the round button at the bottom middle).

It saves the screenshots to your Photos, so you can email them, or just sync over to iPhoto.