Back in 2004, Nattyware conducted research on UI for a customer. The main goal was to find a way to improve productivity by helping users to stay focused.
We found this is might be achieved by making all background UI elements less distractive to a user.
First method to do so is to adjust windows transparency according to Z-order. Attractive but confusing, this degrades productivity. You may use Feewhee for experiments to confirm that.
Second method is to replace transparency with colors fade.
Dusty is a proof of concept prototype made for third method: all unused desktop items become dusty like in the real world. The idea is very simple - the longer an item stays inactive, the more dust it collects.