Siri is the latest cleaver voice command that you can do much more than you can imagine. Of course with Siri alone, there is not many stunning thing that you can do but you can imagine using it will support by other app. With SiriPrefs, you can use Siri to launch App and even setting.
To use SiriPrefs, your idevices must be jailbroken. It is available on Cydia that you can find the source there.
In addition, you can find out guide on how to jailbreak idevices from here. For iOS 4.3.3, you can follow this guide. For iOS 4.3.5, you can follow this guide. And for iOS 5.0.1, you can follow this guide too.