I'm probably harping on this too much, but... I would like to note that "compatibility with Arc/3.1" is a result, not a goal. I personally feel you should drop the whole "backwards compatibility" thing. You already did to a certain degree, but only a little, with things like using stdin rather than (stdin), etc.
There are many ways to achieve backwards compatibility, like a compatibility layer, or having separate runtimes, etc. I think that one of ar's goals should be to create a better Arc. Arc has stagnated, it lacks pg's raw desire to make it better. We're fretting over backwards compatibility when we should be pushing Arc further.
There's tons of languages out there that deal with backwards compatibility, but not too many that try to become better by throwing out backwards compat completely. That's the language I fell in love with: one that tries to be perfect, to a fault. I don't want to see Arc stagnate.