Unfortunately, it doesn't do paren- or quote-matching, and it doesn't interact well with the current arc> prompt.
$ racket -il readline -f as.scm
Welcome to Racket v5.0.2.
Use (quit) to quit, (tl) to return here after an interrupt.
(+ 1 2)
With access to the underlying Racket, one can (require readline/readline) and do (readline "arc> ") to get the proper prompt, but a) this requires redoing the Arc REPL, either by hacking (tl) in ac.scm or by writing something like (while t (prn:eval:read ($.readline "arc> "))), and b) it still doesn't do the matching, which I find an important feature.