I had the exact same problem as described earlier - track pad wouldn't turn on after changing the batteries, not a flicker of the green light, so I figured it was a connection issue. Batteries were swapped with my magic mouse, and they worked fine in that, so I swapped them back and got to work.
It was that same white plastic circular spring on the inside of the battery chamber that was playing up.
I tried the chopstick trick but it didn't seem to be caught on anything
Shaking it didn't do anything
I tried the aluminium foil cube with success, but I needed it in the deep end of the chamber, not near the screw lid as iamsachirock tried.
Came on straight away!
Hope this extra approach helps any one else who has this seemingly common problem.