My original wired Mighty Mouse, purchased shortly after it was released stopped scrolling down a long time ago. With nothing better to do tonight, I decided to open it up to try to fix it. It wasn't nearly as hard as I had expected. The reason it wasn't scrolling down was obvious. There was a huge clump of dust on the down scroller pickup wheel, and it was pushing on the wheel so that it didn't even make contact with the scroll ball. Anyway, if you've got an out-of-warranty Mighty Mouse (this is most definitely a warranty-voiding procedure) that won't scroll, it is likely possible to fix. I found the instructions here. Do note that there are some small pieces and a fair amount of care is required, but it just requires a steady hand, care and some patience.