Version:            (using KDE KDE 3.3.91)
Compiler:          gcc 3.4.3 
OS:                Linux

First of all, this is _not_ a request for a function to minimize every application to the tray.
My idea is adding a "minimize to tray" title bar button, besides the minimize/maximize/close buttons there are. This would make it more clear for users what the close button does. There are lots of applications that do not close when clicking the close button, but minimize to tray instead, for example Kopete and Konversation.
The button should be extra and not a replacement to the close button, but should be located in the upper-right corner by default (on the place of the close button), should be hidden when an application does not support minimizing to tray, and should be customizable via KControl (like the current buttions are).