One comment I've seen reading through various OS X forums about Gimp-Print is that the packaging is important. An experienced Mac person built an OS X package for us that runs a traditional OS X installer, and people appreciate not being forced to use the command line. Even having to type "ports install gimp" -- or more likely "sudo ports install gimp" -- will turn off the hardened Macintosh user community. These folks are even more command-line averse than Windows users.
I couldn't have agreed more with this. On advising fellow OS X users I try very very hard to find alternative solutions to having them do something from the command line. It is simply an obstacle most Mac users are willing to live without.
If you want acceptance on OS X, I *strongly* suggest doing a proper disk image package (a .dmg file is a filesystem image) that installs everything required (gtk, gdk, glib, all the plugins, and then does all of the necessary configuration). I also recommend building a binary package; compiling requires installation of the developer tools (400 MB download) and is quite slow.
..and, might I add, even an installer package is something the average OS X user prefers not to see. The application installation method that is preferred is simply "1. open the disk image, 2. Drag the application to whatever folder you want to keep it in, 3. Run"
I'm not suggesting this could or should be done with the Gimp, I'm just trying to say that if general acceptance of the Gimp as a viable Photoshop alternative is the goal, rather than just an application for the recent switchers coming from unix or linux, the application needs an easy installation procedure that won't require anything other than simple short instructions.
That said, I have 1.3.23 running on my Powerbook, in OS X, and with the Industrial GTK theme and a bit of fiddling with the font sizes, as well as some patience when loading filters (ref. Sven's mail in this thread) it looks and behaves quite OS X like and is more than usable. I'd even go as far as saying it actually looks beautiful, an expression I have not used a lot when comparing X11 apps to "native" apps.
Thus, I agree with Sven on the fact that a full port of GTK+2 might not be a requirement at all.
I think making the installation routine as simple as possible is sufficient.
"A programming language is low level when its programs require attention to the irrelevant."
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