The scroll-ball in my Mac "Mighty" Mouse went completely doolally last week - first it stopped scrolling down, then wouldn't scroll up. Being an Apple product, it's completely impenetrable, so cleaning it is a bit tricky.
Apple's official suggestion is to hold the mouse upside down and give it a good vigorous scroll. Nothing happens. Various Mac forums suggest alcohol or ethanol solutions, or a puff of air from one of those camera-cleaning spray things. Still nothing.
But now I think I've found the answer: spit.
It feels (and sounds) a bit wrong, but covering your finger in spit and then having a bit of a scroll around quickly unclogs whatever was clogging your ball. Simple.