The iPhone 3GS is jailbroken (with a PwnageTool bundle), as well as the iPhone 3G. The iPhone 3G is easiest and more interesting, as anyone can easily add multitasking/home screen wallpaper/battery %.
Jailbreak | Enable multitasking/home screen wallpaper/battery %
Jailbreak | Enable multitasking/home screen wallpaper/battery %