RMSGlass is a new shader, added in RMS 4, and it is part of a collection of physically plausible shaders and lights. It's really easy to use and you can get photorealistic renders with a fairly small amount of effort. For me, two of the more interesting attributes on the RMSGlass Shader are Compute Absorption and the Absorption Unit Length parameter. When Compute Absorption is enabled, the Glass Color is used as the absorption value. It's basically absorption using Beers Law. Very pale colors will show the difference in absorption, based on the thickness of the material best. To make the image appear correct you can adjust the unit used for the calculation using the Absorption Unit Length parameter. Neato!
Check out some of my test renders below