Ok, ok it has been awhile since I have updated this... but anyway, I have been playing (er.. working) with Fedora Core 6 (Rawhide) and they have included a new super spiffy window manager called compiz. You can enable it just by clicking System->Preferences->More Preferences->Desktop Effects.
Anyway one of the new features of compiz is true transparent shadows on a window. So I got thinking and said to myself "Self, what if we can make the shadow move like it is responding to the sun". So I played around in gconf editor and found that you can adjust the shadow in the fly. So I took a look at the desktop-effects.c code and the grandr code and whipped up a Gnome Applet that has the shadow shift from 6am to 6pm. As the "sun" goes from right to left over your monitor (that is the convention I used). The shadow goes from left to right. So in the morning the shadow will be on the left and bottom of the window. At noon it will be just at the bottom and then at night it will be on the bottom and right. So far I have been playing with it on my machine and it seems to work as expected for the most part. If you want to play with it you can get it from here.