wren is half right: at the level of Unixy APIs (and this includes anything that goes on in a Terminal window and anything that you will be doing from Haskell) you use UTF8, but OSX APIs --- that is, Carbon and Cocoa --- use UTF16. So for the purposes of ghc/jhc OSX is UTF8, but if someone wrote a "fully native" Cocoa-based runtime it would use UTF16. (Compare Win32 APIs to mingw APIs; very similar situation.)
I guess the question here is what it _means_ to say that "the standard encoding" is UTF-8 or UTF-16 or anything else. I took it to be an *external* property: "what is the expected/best supported encoding for *user* data". By that criterion, the only standard encoding for Mac OS X 5 would appear to be UTF-8; it did not come with any support for *user* data in UTF-16. If the question means "what is the encoding used by the kernel, or by the native interfaces to the kernel", that gets a different answer. I've now lost track: what did the original poster mean?