I own Apple's Mighty Mouse and noticed one night that it would scroll down but not up. It appears that some people have problems with the Mighty Mouse doing this or not scrolling at all. The problem is small particles get within and under the little ball.
When my mouse did this... this is what I did to get it to scroll up again. I turned my mouse upside down and I got a clean cloth and dampened it and then rubbed it against the ball.
The guy at Apple also stated that you can try the canned air and blow the air at the ball to dislodge any dirt as well.
The problem is the little hole that the ball sets in gets dirt around it and this causes the ball not to move. When I clean my ball I do press down a little bit to make sure the edge of the hole is clean as well...
But if you are still having problems with it and these ways are not fixing it and your a gutsy person you can try the method below. He will show you how to completely tear your mighty mouse apart to clean the hole thing.
Mighty mouse gutting and cleaning | creativebits