Before the release of iPhone firmware 2.2, it’s nearly impossible to take screenshot for the lock screen. I have suggested a workaround in earlier post to use Veency as an alternative way to capture the iPhone lock screen. This works but only on jailbreak iPhone.
With firmware 2.2, Apple finally allows you to take screenshot for iPhone lock screen using the same screen capture keys. You can hold down the Home button and press POWER/SLEEP button. And you’ll notice a momentarily flash. After that, you’ll find the screenshot in your camera roll.