> Moreover, all the popular environments support both GTk and Qt fairly well, and both are cross-platform, so you would actually be supporting more users than with a Mac OS-only program.
By that logic Windows should be first since it has the most users. Also, are you sure that the installed base of Gtk+ or Qt (or both) is greater than the installed base of OS X?
Well yes, because unless I'm much mistaken, Qt and GTk are both supported by OS X so they are both supersets. (This is obviously ignoring Windows, which is also supported by Qt and GTk but is probably difficult for other reasons.)
There are also cross-platfrom native options like wxWidgets. These also work both on Linux and other systems.
Finally, the last StackOverflow survey[1], which I think is pretty representative of the sort of people who would use a program like this, had both Linux and OS X at about the same level (~20% each). So Linux is certainly not negligible.
By that logic Windows should be first since it has the most users. Also, are you sure that the installed base of Gtk+ or Qt (or both) is greater than the installed base of OS X?