Version: 2.69 official (win 64)
How to reproduce: Render the scene (just a sphere with a very bright lamp) and inspect the composite node tree. With the "C" curve completely inverted, the expected output of the RGB Curves node should be a complete inversion of the input, but it isn't. Also try flattening the curve at 0.0, which should make the whole frame dark, but this also does not work.
Comparing the render result to the composite, It seems to only subtract 1.0 from values greater than 1.0, no matter how low the curve is. Changing the extrapolation of the curve has no effect.
The individual RGB channels *do* seem to work as expected on values > 1.0, so perhaps this is not a bug and I'm just misunderstanding the purpose of the C channel? I don't know why I would want it to work the way it currently does, though.