Gambit-C Scheme 4.0 is now available with pre-built binaries (or build from source code). Mac OS X and Windows versions are available with the OS X compiler generating fat universal, PowerPC, or Intel code.

When I use Scheme, I use MzScheme/DrScheme slightly more often than Gambit-C Scheme because of the existence of more 3rd party libraries, but Gambit-C Scheme compiles faster applications and the Snowfort library site now has many canned libraries for Scheme development. Good stuff!