The original iPhone and iPod Touch with firmware OS version 1.x does not has a built-in utility to take or capture screenshot or screen snapshot image on iPhone or iPod Touch, unless user hacks or jailbreaks the iPhone or iPod to run third-party screenshot capture utility.
This is no longer the case in the new iPhone firmware OS software version 2.0. iPhone and iPod Touch user who upgrades and updates to firmware operating system v2.0, which among other features is support of App Store, together with iPhone 3G can enjoy the built-in screenshot capture feature newly added, making the taking screenshot images a very easy process, without the need to download and install any third party or additional applications.
To capture the screen image or take a screenshot in iPhone 2.0, iPhone 3G (by default is using firmware software v 2.0), and iPod Touch 2.0, just press the Home button and Sleep/Wake (On/Off) key at the top of the mobile device simultaneously at the same time.
The Apple device screen will flash and an image will be automatically saved and stored in the iPhone or iPod Touch Camera Roll or photo album.
Update: The trick to take screen capture works also in iPhone 3G, 3Gs and 4.