MacBook Pro 2.53 GHz, 8GB, 15 In. Late 2008.With 500GB ,
Suddenly the track pad started "holding"not letting go after taps, etc.
I removed the battery to check thetouch pad, and, in a hunch, just connecetd the exterior power, no battery inside,
The problem went away: The battery seems a tiny bit "bloated" causing pressure under the touch pad, thus 'engaging' the lock for either track pad or mouse.
The battery is old, and possibly deformed, just that 0.5 mm or less that causes the trackpad to engage.
I will keep an eye on deformation and pressure, especially in intense applications and warm environment.
Solution: new battery, no battery, or ignore and get new touch pad. Battery about $ 110.00, pad, $250.00 plus labor.
I hope this helps.
In my case, it was a swelling battery: I have an early 2008 MacBook Pro, recently upgraded from Leopard -> Snow Leopard -> Mountain Lion. But my recent very erratic trackpad and keyboard was indeed caused by a swelling battery. The "Service Battery" alert appeared in recent weeks, on a battery I usually leave plugged into power. After reading other postings, I checked the battery, which was indeed swelling noticeably, interfering with the trackpad and keyboard. Switched to my alternate battery, which still reports 87% health, and all is better now.