This is under the trackball of a Blackberry Bold 9000 after about two years of light-to-moderate use. To get some lovely detail of the grime, you'll want to view that image full size.
It looks almost uncannily like a birds nest aside from the scale - it's just a few millimetres across. Perfectly shaped to the underside of the roller, it kept it's form as I yanked off the protective plastic coating. I'm not sure what material kept it stuck down during this but it certainly didn't wipe off easily. I'll be disappointed when the 9000's are retired as the trackpad handsets don't have this kind of character (I haven't had a single one in for cleaning yet).
The best part: The roller was still working perfectly, it was brought in because of antenna issues.