It seems to be an actual Lisp-1 dialect, not just Scheme-like syntax. In (Lion's version of) application.sb, there are a whole bunch of lambdas and letrecs flying around. They even define a macro.
Interesting, I never really dug too deep into it, aside from making a couple of changes and standard entitlement stuff for applications. application.sb looks like a complete nightmare to maintain.
It's specifically TinyScheme, with one every-so-small modification to the parser.