**urbanarmitage** I did some quick experiments and I've found that it can be done, but there are some things to consider.

First, The ambient colour of one object may negate anything seen ont he other.  The reason for this (I suspect) is because "Ambient" is supposed to imitate a glowing effect.  In other words, it is supposed to make an object act like it is illuminated (though in a very weak way as shown by the fact that tricks like this are needed for it to actually illuminate anything other then itself.)

In other words darker colours show the gather effect better then lighter colours.

Another thing to consider is what your raytrace bounces are set at in your Render settings.  I set mine to 4 for the test.

Also, if we set up in the way that Little Dragon showed at the beginning of this thread, there will be no appreciable effect.  The Gather Node plugged into the Ambient Colour seems to only produce Black when the colour is set to anything other then White.  I don't know why this is.

So, to get around this I plugged the Gather Node into Reflection Colour.