this is going to sound stupid but test your touch screen zone by zone..
it is a battleship game ( i will admit fun to play ), but it zones off the screen into squares and this will let you see exactly where the screen is unresponsive. If it works through-out then it is a software based issue and another full restore may be in order
Mine did this yesterday. It is right where many dialog inputs are and basically the whole top row of the keyboard. I thought sending it in to Apple would be an easy trip down to the Apple store, but no, they are making me mail it in and charging me $30 to use a replacement while it is being repaired. I'm not happy at all!