That's true. Programming languages take years to evolve, not months. Moreover, Arc is in the language design phase. As of today, the syntax and the semantics of the language are more important than libraries. For example, I'd like to see something like Kenny Tilton's cells (http://github.com/stefano/cells-doc/) nicely integrated within the core language.
I would suppose that since a) this language is not used in anything "important" yet, and b) it's open source; yes, it can be in the design phase. I should think that the design phase persists until, for some reason, the language becomes "formalized", or until it is impossible to change anything major without ruining things for a lot of people. At that point you can still "design" the language, but since it has a generally understood "style" and so forth, it won't be able to change too much unless you make a whole new language.
What do you want to be designed? One of the major problems about "designing" a new lisp is that, since anyone can "do it" in their spare time, they don't see the point. Maybe they're right. ^^
Sorry for all of the quotes; it looks kind of funny, I'm sure.