As the Mighty Mouse Repair Guide notes:

After the release of Apple’s Mighty Mouse™ many users found that the scroll wheel on the mouse easily became clogged and difficult to use - especially when trying to scroll down.

 Apple’s own site suggests using water to clean the scroll wheel but this method has proved either temporary or unsuccessful for those that have tried it.

The repair site goes on to give instructions on how to disassemble the mouse to clean the scroll wheel using a razer blade or scalpel.

Good to know that Mighty Mouse surgery is available as a last resort, but I found an easier alternative that worked for me– a dollop of Purell.

I applied it to the wheel, spun the wheel, waited a few minutes for it to dry, and was good as new.

I’ve used a similar approach to fixing the trackwheel on my RIM Blackberry in the past, with no ill-effects.