Window modal only for other window

[DaVinci <bombadil wanadoo es>]

 Hi.

 I have thought it could be a good thing to have a function that makes a
 window modal for an specified parent window, but not for rest of
 application.

 Has anybody thought this? Is it too hard to implement? Do exist any reason
 for not doing it?...

 I am curious ;)

 Greets.

                                                David