I just saw these symptoms on my macbook and it turned out to be my magic mouse in a bag close by with the button pressed down. Take a quick look and see if you have any wireless mice in range that might be causing the problem.
In my case, I guess it's the battery is swelling up.
I have a MacBook Air, using the trackpad I can still 'tap to click' to select something on the screen, but a real physical click is not possible anymore. You need this for example to drag and drop something, to move a map in Google Maps etc etc.
No external mouse in the neighbourhood here.
If I push the touchpad very hard, I can still 'feel' a small physical click (nothing happens though), however I checked on a newer MBA, there the physical click feeling was still clearly there.
I guess I'll have to replace the battery (myself). It's almost 3 years old...