Why oh why is Gtk2Hs developed using CVS?!
It is mostly due to history and inertia; Gtk2Hs has been in development for a lot longer than darcs has been around. We use SourceForge for hosting and it only provides CVS. That is not to say we would never consider changing, but we do not have any current plans to do so.
darcs is indeed an excelent source control system. In particular, it lowers the barrier to entry for outside contributors