I don't know whether this is the issue, in your case, but I'll tell you what it was, in my case.
With my pre-unibody MacBook Pro, the trackpad lost it's clickiness when the defective (removable) battery underneath the trackpad swelled up, preventing the clicker from working properly. The solution was a battery replacement. I had two defective batteries, and the trackpad losing its clickiness was the first symptom of failure of the defective batteries, in each case.