I have a 15" Macbook Pro, and it's had this issue for as long as I can remember. Randomly, it will scroll the screen downward. It doesn't matter which application I'm using (browser, Openoffice, etc). And it doesn't matter if I have a mouse plugged into the USB port or not. I'm running OSX 10.6.2, and prior to that I was running Leopard. The upgrade didn't fix the problem. Any ideas would be MUCH appreciated!! Thanks.