My work laptop has a touchscreen. I've never used it, but other people use it by accident fairly often. Usually only once each though, the look of shock is sometimes even worth the fingerprint :D
I've never understood people who do this. You can point at the screen, tap with a pen, take the mouse or keyboard and move the cursor, etc. but surely it's bad manners to splodge your finger on someone's screen.
Touch the computer, sure, but please don’t touch the screen with your filthy grease fingers.