If the device is covered under warranty I would not try to fix it.
I have had this problem with two different 8900's. The last one was not covered under warranty because I had unlocked it - to use a T-Mobile phone on ATT. T-Mobile, ATT, and RIM all refused to repair or replace it.
Anyway, the fix was to replace the trackball flex cable, which is the board that has the switch that gets pressed when you push the ball. It is also the interface for the other trackball functions. If the trackball is removed the board can then be pulled out. The connector on the main board first has to be disconnected for removal. Then the board and the cable fished out. The new board goes back in like the old one with the cable threaded through and connected on the other side of the main device board. I found my new replacement board on eBay for $20.