Well, it obviously seems like it is behaving like the trackpad button is stuck. Try a few hard clicks and releases to try to "un-gum" it up. Has anything been spilled on this machine? You can also try removing the battery and applying a little pressure behind the button to help release it.
If none of this works, and you're willing to perform surgery, you can take off the top case and check the button and the keyboard/trackpad connection cable to the logic board.
Steve