It happened to a friend of mine - I googled the problem, and the solution I read at the time was, in a nutshell, scroll violently! So I showed the scrollball no mercy, and scrolled with anger, with energy ... there was so much scroll action I could have been in some kind of historical Roman docu-drama. Anyway - to cut a long story short, it did the trick - it must have dislodged whatever had become stuck in its mousey innards, and it's been working happily ever since as far as I know.

Getting a whole new mouse, free, is a much nicer outcome though.