In fact arc3.1 even works on Racket, the new PLT Scheme. Only thing is that the command-line "racket" prints a newline after the "arc>" prompts, for some reason. But you can open as.scm with the editor DrRacket (as you could with DrScheme), set the language to be "Pretty Big", and hit Run; it will work.
For some reason, now I don't notice any issues with the "arc>" prompt in "racket" either. And I don't think I'm doing anything differently than I was before. ...I am forced to conclude that, when entering things into the REPL, I held down the return key long enough that it accepted an extra (blank) line of input. This explains the behavior exactly. Strange that I should have done this several times in a row... and how embarrassing. Oh well. At least now I can give racket a clean bill of health.
Yeah, I'm not trying to say you shouldn't use it for production use :)
They're opposing perspectives. As a user of arc I'd throw it into production. At the same time, from PG's perspective I'd want to be conservative about calling it production ready.
I suspect arc will never go out of 'alpha' no matter how mature it gets, just because PG and RTM will not enjoy having to provide support, or having to maintain compatibility.
 With some caveats: treat it as a white box, be prepared to hack on its innards, be prepared to dive into scheme and the FFI. And if you're saving state in flat files, be prepared for pain when going from 1 servers to 2.