http://www.engadget.com/discuss/scroll-wheel-not-spinning-freely-anymore-here-s-how-to-fix-1lr/
When my scroll wheel stopped spinning freely, I was disheartened, as I really enjoyed the feature for very long webpages. Looking around online, it seems many others have had the issue. There are usually two reasons why it happens:
1) You have a piece of debris or residue stuck inside against the scroll wheel, keeping it from spinning properly.
2) The rubber strip in the middle of the scroll wheel has come loose a bit and is rubbing against the wall
The first issue can easily be fixed by cleaning the debris/residue.
The second issue can be easily fixed as well. I had to do this myself, and I don't miss the rubber at all. Supposedly the rubber is to help maintain grip and traction for your finger when spinning. However, I don't notice a difference either way.
Simply use a scissor or small knife to pull the strip up, and then cut it. Then pull it out and discard. Voila! You have a freely spinning mouse for as long as you have the mouse. I've had the mouse for almost 2 years now with no issues.

Mine is stuck right now and its been a pain for a couple days. I think there is some crap in there, I don't want to cut the rubber off though. Maybe I can clean it with a thin piece of something.
Ever happen to you?