Neat idea, but I'm seeing a couple issues with it:
1. I have a multi-monitor setup where the primary monitor is in landscape mode (1280x1024) while the secondary is in portrait mode (1024x1280). Even with the multi-monitor support turned on, it doesn't seem to dim anything on monitor #2.
2. If I change to showontop=1 (to always show the always-on-top windows), my task bar goes dim when the app first starts, and doesn't undim until the first time I click on it.