My family has 2 Core 2 Duo MacBooks and while they were under warranty (3 years) I had 3 or 4 top cases replaced because of this problem.
I thought that it was probably happening because the laptops had been packed too tightly in my bag for too long and that that was squashing the trackpad etc.
Anyway each time the top case was replaced the problem went away for quite a while. They are still going well now years later and I've never replaced the batteries on either laptop.
So maybe the top case on that model is prone to damage as well as the possibility of battery swelling causing the problem.