The scroll button of my Blackberry Pearl 8100 does not allow me to scroll upwards anymore. I can still scroll downwards and sideways, and I can still use the button to click.
Have a look here as there may be dirt in the trackball mechanism. . .I had the same problem, cleaned it out and it works fine now. It can be a bit tricky putting it all back together again but if you take mental notes (or pictures even) of how everything fits together you should be ok.