Is there an iPod app to easily switch wifi on and off? Is there an app whose only purpose would be to change the wifi prefs, by starting or stopping the wifi on iPod Touch? I would like to have an icon on the home page, and a simple click on it will switch the state of the wifi.
Through the App Store? No. Apple's guidelines don't allow apps to turn on or off Wifi.

Jailbroken? Yes. There are program such as SBSettings that will do just what you want to do.