I have a MacBook running Ubuntu 9.04. Every once in a while, the mouse moves by itself. I always assumed it was my palm grazing the trackpad. If you have VertEdgeScroll turned on (which is the default), it's really easy for your right hand to touch the right side of the trackpad and cause your window to scroll. I figured OS X was just better at palm detection.
However, I recently bought a really nice external monitor, keyboard, and mouse. I was sitting reading some code. I was nowhere near my trackpad. In fact, I wasn't even typing or moving the mouse, although my hand was resting gently on my mouse. (If you're in the mood for a good conspiracy theory, keep in mind that it's a Microsoft mouse and an Apple keyboard.) All of a sudden, my mouse started moving. It's not a bluetooth mouse. None of my friends are around to be playing tricks on me (that I know of). It moved three inches right, and then down and to the left, and then all the way to the right. What the heck?!?