Apple has recently filed a patent for translucent windows that automatically adjust the opacity as information is updated (or not updated): "overlaid, Information-bearing windows whose contents remain unchanged for a predetermined period of time become translucent. The translucency can be graduated so that, over time, if the window's contents remain unchanged, the window becomes more translucent. In addition to visual translucency, windows according to the present invention also have a manipulative translucent quality. Upon reaching a certain level of visual translucency, user input in the region of the window is interpreted as an operation on the underlying objects rather than the contents of the overlaying window."
News source: MacNN