Hello, I found the following information extremely helpful when wanting simply to allow my iPhone game to be played on the iPad (without recreating all of my artwork for a bigger screen):
Simply, mark your project as targeting the iPhone only. This (although not obvious) will allow the app to be run on an iPad as well! It will be displayed in the center of the screen on the iPad. Better yet, Apple even provides a "2x"/"1x" button by default to blow up the image (although keeping the same lower-res resolution).
No extra work required, just mark this setting and design for the iPhone
Hope this helps anyone wanting to extend their app to the iPad.
BTW, does anyone know if something similar will work for the Retina Display??? (I am guessing that the app will be "blown up" to fit the screen, with the lower-res resolution)