Is there an app out there that will allow me to see who's calling my iPhone from my iPad? This seems so simple, yet there doesn't seem to be anything in the App Store, leading me to believe it's not possible.
As you figured out yourself, apps allowed on the AppStore can't do that.
Worst, I'm not even sure something like that exist for jailbroken iDevices, since I did not find things like that.
Also related : iPhone ring notification on the computer.