I mentioned recently that I was the new owner of the UAC UI (from the dev side).  Well, today I found that Raymond Chen's blog has brought up the topic of what the colors mean.  I hadn't even really noticed that the dialogs changed much before taking ownership of the code, and due to the ease of repurposing attacks there is little comfort in seeing that a particular item is signed.  It's a good first pass, but I definitely think we have room for improvement in creating valuable distinctions and conveying that information to the user.