At the moment the production release is the current "trunk" on SVN, which is packaged on exchange.nagios.org. There isn't an official "development release," just the devel SVN repo.
Part of the request for a new interface was a nagios "facelift." Some of the icons are new, while the old list is still maintained and ship with V-Shell. I appreciate the feedback, although I'm not sure including a new icons directory really qualifies as a bug.
I appreciate you posting the instructions for linking the icons to the Core interface, as well as the feedback!