Looking further into Snow Leopard's fonts, I've found a couple of interesting refinements:
The character repertoire of Geneva has been expanded considerably. Ranges added are Greek, Ogham, Cyrillic Extensions A and B, plus two blocks from Unicode Plane 1, Ancient Symbols and Old Italic.
The new monospaced font Menlo has a broader than expected script coverage, including Arabic, Greek, Cyrillic, and Georgian in addition to the normal Latin. Lao is also present, but about 20 characters seem to be missing.