I cannot recreate the problem some of you are facing - i have been jumping up and down with the BB in my pocket with keypad lock, and it still seems to ring fine. I also tried putting the BB into standby mode by pushing the mute button for 2 seconds, and did the same thing.
Only way I could recreate the problem was to physically scroll the trackball continously by hand while the call was incoming...
Perhaps some trackballs are more sensitive than others - my settings are 70 each for horizontal and vertical sensitivity...