At least the Carbon port has an issue with drawing xpm icons (e.g.  
typical tool-bar icons) when icons for the "disabled" state are  
specified.
Key definitions for the tool-bar map allow for a vector of four image  
descriptors to be given.

This works fine for xpm on X, but not in the Carbon port. Here, if a  
"disabled" state is given, it is not displayed - see screenshot.  
Interestingly, png images may be specified here and things work fine  
then.
Have a look at the test case below to demonstrate.

(As a side-note, png masks don't seem to be supported, not even with  
bitmasks rather than a full alpha-channel. Is that correct? that's  
the reason why I find myself using xpm in the first place).