Yes, arc is still under development. PG doesn't really tell us that much about what he's working on, but periodically a new version will be posted on the website.
There's also the community version hosted on github, Anarki, which is under fairly constant development, and adds a few nice features over vanilla arc. However, PG is designing his language to only include what he actually uses, so most of the changes are unlikely to be copied over to the official version.
As for specification, since the source code is extremely short (only 2500 lines) and most of that is basic library function definitions of 5 lines or less, PG claims that the source code is the spec. There are some community created documentation sources, but the source code is pretty clear.
