How-To: Take iPhone or iPod Touch screenshots!
Step 1: Briefly hold down the power button on the top of your device. While it's down, press the ... uhh, well, the only other button these things have. The one on the front. And that's all! Your screen should flash momentarily, and your screenshot should be saved in a new folder under Photos.
Appropriately, it's called Saved Photos. When you want to get your photos from your iPhone/iPod to your Mac, without having to mess around with syncing in iTunes, you can just pop open Image Capture -- it comes standard with OS X -- and pick the ones you want.