Wow, I got a new MX518 from Circuit City today. The scroll wheel is 100% different. I think I still prefer the MX510 but it has been completely overhauled from the original MX518. It works oh so very much better.
1 - I had an actual defect in my old scroll wheel
2 - The scroll wheel feels much better and is smoother.
3 - The mouse's movement is way, way smoother. I'm not sure if I had another defect in the sensor of my first MX518 or if the new one is a new revision and improved.