Adam Russell wrote:

We had a leak from the 2nd floor tub/shower down through the ceiling
whenever someone used the shower. It turned out to be the plate at the foot
of the tub (on the tub wall) where the lever is to close the drain. I
noticed that the spring that holds the ball of the lever up against the hole
was loose and not functioning, so putting that spring back in place
stoppered up the lever hole. But I always thought that that plate also
functioned as overflow relief. Isnt all water that goes in there supposed
to flow right to the drain? I'm wondering if I may have another problem
hidden in there.


You are correct; it is an overflow.

There is a rubber gasket *behind* the tub,
between the drain fitting (called waste and overflow assy)
and the back of the tub. You can access the gasket from
the tub side by removing the trip lever faceplate (2 screws usually).

http://www.plumbingworld.com/waste_and_overflows.html
scroll down to Trip Lever Bath Waste

Jim