I acquired a copy of Windowblinds and couldn't find anything to do that
It's strange...there are something like 20 billion programs for Windows XP to allow you to manually set a window's transparency, and 5 billion of those will even remember the transparency, but there doesn't seem to be a single one that will do it automatically based on whether the window is active or not!
For now, I'm using
Y'z Shadow which can give windows a different shadow based on whether or not it is active. It's neat, but it doesn't address the reason I was looking for this app in the first place. I want inactive windows to "fade away" so that, for the most part, the only thing grabbing my attention is the window I'm using. Is that so much to ask?