Now hopefully someone will get around to writing a Gtk backend for Quartz (the MacOS X windowing/drawing mechanism). (There are already backends for Win32 and frame buffers, so it shouldn't be impossible.) Then it will be possible to run GIMP on MacOS X without an X server.
Another thing I'd like to see would be some Python bindings to Cocoa/AppKit. (Given the nature of the Objective C runtime, it may be doable.)