When my Might Mouse™ from Apple stopped scrolling up, I took a few minutes and found a fix on Apple's support discussion pages. Here it is, to help others fix their mice up, real nice.
From Kitchissippi's post:
"Just hold the mouse upside down and firmly press and scroll the ball for a few times. There's gunk in the sensors (dead skin cells, etc.) and it needs to be cleared — remember the old mice with the rubber ball? Well it the same except the ball is now on top! Next, I take a scrap of paper, fold it in half, and run it along between the ball and the hole to scoop up the loosened gunk. Short of taking the mouse apart to clean it, this is the easiest way to get the scroll ball working properly.
Hope this works for you."
It did! I took a business card and gently scraped around the top of the hole where the track ball sits and lo and behold, lots of gunk came out. Ewwww. Thanks!