MuseScore for Mac is out of beta
MuseScore for Mac OS X is out of beta since the release of MuseScore 0.9.5 on the August 15th 2009. The feature list for Mac OS X is the exactly same as for Windows and includes Midi input, output to wav, flac, ogg and more. Download link: http://www.musescore.org/download/musescore.dmg
MuseScore is available for PPC & Intel as an Universal Binaries package. It has been tested successfully on MacOSX 10.4 and 10.5. In case you successfully tested MuseScore on Snow Leopard, don't hesitate to leave a comment.
For developers, instructions for building MuseScore on Mac are available.