With the new 2.0 firmware, you can take a screen capture using an easy key combo. Just hold down the home button and press the power button briefly. If you hold it for several seconds, the phone will soft reset, but if you just press and release, you’ll see the screen flash and an image will be stored in your photos folder.
This ought to come in handy if you’re doing any programming, web-based or native, on the iPhone. Nothing beats having a screenshot during QA to help document problems.