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.
As an alternative, if you're running iOS 4+ you can go to the Settings icon and then there's the 'Flight mode' option at the top which does just that. Not as a simple as you were hoping for, but it is just a click and a swipe.
I have a Linksys WAG160N which my iPod disagrees with so I frequently have to reset my wifi this way, and it's much quicker than the old iOS 3 workflow.