I think that the best way would be to basically copy the UI of the call popup and switch the two buttons with a slider (just like [bleh] windows mobile including the answer/ignore being the backround of the slider). Also making it only portrait wouldn't be bad since when we talk the phone is in portrait position and I find it quite annoying that after finishing a call the phone turns from portrait back to landscape (at least an option to fix one of the positions would be great).