Hi, Sorry for the late response here, I just saw this thread.
Err, from the Web site: "The port is based on GNU Emacs 20.7, which is one behind the latest version. Emacs 21 added mostly UI features, not core editing functionality. The UI is nicer though and furthermore some Lisp packages have started requiring Emacs 21. So there are plans to update this port to version 21 once the 20.7-based version is fully stabilized. Contact the developers to get involved." -> it is based on a version of Emacs that Debian no longer supports If you want to see Emacs for GNUstep in Debian some day, please talk to emacs-on-aqua people and ask them to work for its inclusion in the CVS trunk (there already exists a Carbon port).
As mentioned above, this is planned. However, there is some further work to be done stabilizing the Emacs-on-Aqua-specific code that is best done before attempting the port to Emacs-21/22. Furthermore, the GNU Emacs CVS trunk itself is in an unstable state, and not necessarily optimal for beginning a port attempt right now. So it will be a while until Emacs.app will be updated to the latest GNU Emacs.
The immediate goal of Emacs.app (emacs-on-aqua) however is to produce a solid emacs-20 version that can run under GNUstep with the UI advantages that provides. How exactly is accomodating a version 20-based Emacs on Debian problematic? Emacs itself has a version-numbering scheme for lisp directories to avoid conflicts, and I imagine Debian as well has to deal with the problem whenever there are upstream version changes. (As will soon happen with GNU Emacs 22.)
thanks, Adrian