Let me try from a different angle. There's some settings made by user, like font size, or toolbar icon size. The unit does not matter there.
I believe that in a good GUI library, you should not ever need to use any dimensions for windows dimensions, but just allow the layout code in the library to take care of this. And since you don't have to specify dimensions, it does not matter if they are in pixels or not.
I do think that every now and then, you do need to specify dimensions. For instance, when specifying a button's width (in case you want it fixed). Best, John -- John Torjo, Contributing editor, C/C++ Users Journal -- "Win32 GUI Generics" -- generics & GUI do mix, after all -- http://www.torjo.com/win32gui/surfaces.html - Sky's the limit! -- http://www.torjo.com/cb/ - Click, Build, Run!