This has been noted before. I have seen a couple proposals to add significant whitespace to Scheme which I can't remember the details for offhand, and I believe was also the basis for the Pliant language's syntax.
One such proposal that I like is "sweet-expressions" at http://www.dwheeler.com/readable/. That page also has a rudimentary set of implementations for some Lisp dialects, including Scheme. It's too bad that new Lisp dialects never seem to take notice of these features.