Joachim Ganseman released the first MuseScore alpha for Mac. Quote from Joachims post on the developers mailing list:

I've compiled a first MuseScore for Mac version and put it available for download. A few very important notes however:

• Requirements are Mac OSX 10.4 or higher, and a G4 processor or higher or an Intel processor. It is compiled as universal binary (32-bit) and tested to run on a G4 10.4 system and an Intel 10.5
  system.
• It is based on revision 1685 of the trunk.
• this is an alpha version and is NOT suitable for production environments AT ALL. Really. It contains loads of issues and bugs which I know of but for which I need way more time to solve them. Only reason that I make this available already is the MusikMesse, people over there might be interested to know that a Mac version is really on
  it's way.
• executable size is about 80 MB

List of known issues:

• crashes on shutdown (on my system at least), seems to be something with the QApplication destructor failing to end properly or waiting for some thread to yield which doesn't.
• synthesizer (playback of score) does not work or is not active yet.
• layout of some tabs in the preferences menu is still messy.
• tooltips often have the wrong length -> probably tweaking the normal text fonts is necessary as it was for the music fonts.
• the splash screen does not display.
• svg images for some reason did not work. I'm not yet familiar with the resources system in XCode and Mac Application Bundles, I guess the problem is because I don't use that properly. As quick fix in this version I converted all resources to xpm format, which does work.
• "open recent files" has some problems, could be that case sensitiveness might be an issue here, not sure of that.
• this version I put online is not linked to portaudio. I didn't see the use really since the synth did not work even when portaudio was linked in.

With this list of known issues, one might wonder what does work:

I invite people to test it and let it know if anything is not right that is not in the list that I already mentioned. Then we can compile a workable TODO-list which can be used to work towards a future 'real' release. For solving the problems I already mentioned, I'll probably compile a debug version of my own in order to be able to do some decent debugging. The software as it is now is really fragile, it's not a release but more a partially functional 'proof-of-concept'.

Compilation options were: base SDK is 10.5, deployment target is 10.4. In this way any 10.5-only functionality is "weak-linked" into the project, which means that the executable will use the most recent versions of the framework when possible but will only use functionality which is certainly available in 10.4 already. It was statically linked against Qt 4.5.0 and uses static linking for all the other dependencies too at the moment.

You can help speeding up the release of a stable version for MuseScore for Mac, by submitting your feedback in a comment to this post.