Cross platform way to disable interface when "dialoguing"

[Ken Corey]

Has anyone read up on what would be the "right" way to do a 
cross-platform indication that the main application window is not 
accessible just now?

I'm thinking of putting an graphic with a gradient and alpha channel in 
front of my interface, so that the style is reminiscient of an iPod, at 
least on the PC and on MAC osX.

Of course, there's no point on an Apple iDevice, because they all 
already do that.

-Ken