Hi Nathan,
Arsenal is indeed under active development. Glad to hear you might be interested in checking it out, we'd love to get more contributors and users.
We're currently running Arsenal in a studio with about 400 users and a farm of 9000 cores. There are several other studios using it in some capacity, with the majority of development direction being driven by us ( Dr.D Studios ) and Blur.
Let me know if you have any specific questions regarding the roadmap.
cheers,
-baz