Expensive but already built for you: http://www.liquidware.com/shop/show/TS/TouchShield
Just a digitizer (touch screen film): http://www.liquidware.com/shop/show/TSC/1-4+Touch+Screen
Just a screen: Color LCD 128x128 Nokia Knock-Off - LCD-00569 - SparkFun Electronics
Thread discussing how to use one: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1205107562
Andrew