BY THE WAY, you get an incorrect result if you apply a specular shader to planar geometry.
Some time ago I had a very interesting discussion with Thomas (aka Lycium): he told me to try and use only the specular shader, instead of the Thing Glass. He strongly supports the correspondence between reality and modeling, because this points the rendering engine's development unidirectionally towards realistic algorithms and less to biased shortcuts, so the result can be more real-like. A good exemple can be the new "architectural glass" algorithm.
To make a long story short, always model volumes, because glass can be only tridimensional.