Mighty Mouse unusual problem

A couple of days ago I had the whole 'scroll ball wont go down' problem but I think it's OK now.

Anyway, now, sometimes, for no reason, the cursor moves to the top right of the screen. It moves up and right first, then along the top to the right corner.

At first I thought it was my university educated brother doing it through his Wintel, but it's happened about 4 times now and I seriously doubt he would be bothered doing this at all, never mind 4 times.

Does anybody have any idea why this is happening. It usually happens while I'm lifting the mouse up.

Are you using a mouse pad? when you say it happens when you lift the mouse up, it sounds, that you are not using one and it would just be reflection. I wouldn't worry about it, if it doesn't appear while you have it on the table. how often do you have to lift up a mouse anyway?