I still use high bitrate mp3's, but I do have some shn stuff. Seems like Monkey's Audio is pretty popular, but the Mac version is still under development. If you know C++, you can assist.
Darwin is the open source version of OS X...sorta. It doesn't have any of X's gui and I believe some of the APIs are missing as well. Darwin also happens to be available for x86, fueling rumors of OS X for x86. I would just try it - if it doesn't work, it doesn't work.