T-Mobile is expected to offer free replacement trackballs for T-Mobile customers with BlackBerry Pearl or BlackBerry Curve (8100/8120) smartphones starting on February 15.
There have been many "issues" with trackballs on BlackBerry devices such as the BlackBerry Tour. In fact the next version of the BlackBerry Tour due from Verizon soon, will have a trackpad instead of a trackball.
In order to get a free replacement trackball, you will have go into a T-Mobile retail store. The list price for a new trackball is around $10.00. It's not just the plastic ball, but the electronics that surround it.