OK so I've recently started delving in to XSI 6.01's features (after being stubborn and sticking with 5.11 for most of my final year project) and thought I'd test the Physical Sky that was added with 6.
To start off i created a simple scene with a simple poly grid for the ground (with a Lambert material added) and a poly sphere with a very simple glass setup i created. Here is the rendertree for it;
And just to test how the glass works using Final Gathering and HDRI mapping i tested it with a simple photography of mine for the environment (instead of using physical sky in the pass shader tab> environment add 'Environment instead' and apply an image, preferably a HDR image). here are the results of that. Any Questions don't hesitate to ask :)