The Blackberries had the capability of easy trackball assembly removal and cleaning; it was just a matter of using my fingernail to remove the chrome/plastic retaining ring and then turning the device over and dumping the trackball out for cleaning.
I do not know if that is possible with the Eris.. but I hope so because I've noticed mine getting "jumpy" now at times, behaving similar to the Blackberry trackball when it got pocket lint in there, dust etc.
I'm not about to try prying the thing apart unless real instructions can be found somewhere, if real instructions for that procedure exist at all.