make sure your screen is clean
1) What kamas said - clean your screen. dirt on the screen sometimes makes the touch sensors go crazy.
2) Sometimes people get this for no reason. It usually just goes away. Restart a few times. Get App Killer and make sure you don't have some app running in the background causing problems.
3) Does the problem go away when you are plugged into a power cable? If so, then it might mean your battery is beginning to die out. If in replacement period get a replacement now.