I recently developed a scrolling problem with my Mighty Mouse. It started to only vertical scroll in one direction. I reinstalled the software and it did not help. So, I called Apple tech support. While waiting on the line, I decided to press quite hard on the scroll button and try to scroll. That cured the problem, after that it only required the normal light touch to work properly. I do not know what was the cause of the problem, maybe dust(?), in any event it is working fine now.