What would be especially neat would be if you'd keep it
non-platform-dependent, so the non-mac crowd could use it too. Maybe
the backend in Ruby/perl/python or even Java/C/C++ so it's portable,
but the GUI done up in pretty Mac OS X (aqua?) happiness.

I'm most tempted to do it coupled with SQL, but that brings in a bunch
of added dependencies that are uncool. Since you're already an emacs
user, have you considered doing something in elisp? Maybe you could
couple it with the beautiful EmacsWiki mode. Personally, I think I'll
take a stab at writing something in Java, mainly because I'm trying to
learn it.

I'm just rambling here, jotting down ideas as they come to me. I'd like
to watch this in the making, if you'd do the hacking publicly. Good
luck!