Well, there are schemes that compile to machine code, like Chicken Scheme, but I guess the ultimate goal is a from scratch implementation. It's wise to postpone that until the language design stabilizes, though.
MzScheme does have a JIT compiler that produces machine code. One of the reasons to use the recent version (372) is that in version 370 it switched to a precise GC which is faster and more stable. (BTW, Chicken compiles to machine code when used in batch mode.)
