I haven't seen an app like you described. There are apps that allow you to tether a laptop computer to the camera with USB, and then control the laptop with your iPhone or iPad, via wifi. The only reason I can think of that we can't tether directly to and iDevice is that Apple only allows programming API's from Apple, no third party API's allowed, and they don't have and API that will enable this function. (API = Application Programming Interface, an interface that allow programs to talk to the device hardware.)
Doesn't appear to be much hope in this area, sorry. I would love to see this type of app happen myself.
