Yeah, I think that's right. And I think Arc should go the path of informality, for a couple reasons. First, it's dynamically typed, and duck typing (or informal typing) has historically worked well with dynamically-typed languages (with Ruby, Python, and maybe Smalltalk). CLOS also tends to that side of the spectrum, although I'm not sure how explicitly it embraces duck typing.
Second, and I think more importantly, duck typing gives the programmer more power, in exchange for more opportunity to screw stuff up. This is very much in line with Arc's philosophy.
Using formal interfaces, both the people writing the polymorphic code and the people writing the polymorphic objects have to explicitly code polymorphically. Using informal interfaces, only the people writing polymorphic objects have to be explicit. The other people can* be aware of the polymorphism, which allows powerful stuff like Ruby's Enumerable module, but as long as the objects behave correctly ("quack like a duck"), you can use pass them to code that doesn't expect them at all and they'll still work.
* I know less about Smalltalk than I want to, so I don't know how much it makes use of duck typing.
