My mighty mouse scroll ball was fine for a month, but until recently scrolling up won’t work. At first I thought I was a software issue, but after updating everything and restarting osx, the issue remains. If that happens to you, don’t trash your mouse yet. Do this.
Try pressing down the ball hard, then roll the ball a little in that position. Now try and see if your scrolling comes back. This works for me.
Best way to explain this is to open up the mouse. Anyone?
ref: http://hints.macworld.com/article.php?story=20060210115417864