Yes, I think Arc intentionally supports only ASCII just to not bother with Unicode issues as of right now.
Anyway, I can't see how Unicode can break in Arc. I'm not a Lisper, but I think you can't extract 1 byte from an Arc string (since it's just a MzScheme string), but 1 char instead. That's a different concept, because in Unicode 1 char can be formed with 1, 2 or more bytes.
