i also had some problems with my Mighty Mouse where the scroll ball stopped scrolling down 🙁
i did a quick research and found this link from apple where you’re told how to feedclean your mouse.
and yes, it really worked, i took a wet cloth rubbed the nipple of the mouse in all directions while holding the mouse upside down and voil? – it works again.
if i get problems again in the future or if this doesn’t work, you can try to repair the mouse by dissasembling the enclosure. How this is done without breaking the mouse can be found here