I've had that problem on several MBPs I've had. I took one in and Apple didn't feel it was a problem since it didn't affect functionality....well, it is a problem for me since I use the button all the time and excpect it to feel exactly the same each time I press down, which it didn't. Sometimes it was ok, other times it was in two stages and sometimes the first stage was loud and metallic-sounding. Anyway, here's my solution:
1. take out the battery
2. put your MBP at the edge of the desk so that the battery area sticks out and you can put your fingers underneath.
3. Put your finder right underneath the trackpad button and press gently while clicking. Try different spots and see if one of the spots makes a difference. Starty with light pressure and, if that doesn't help, apply a little more and continue to click.
4. If you can identify one pressure spot that alleviates the problem, apply a piece of electrical tape in that area. You might need double layers of tape.
The key is to just experiment in order to identify the right spot and that you do this with the keyboard facing up so that gravity doesn't hide the problem. This worked well for me both times - hope it does for you too!