According to http://wiki.blender.org/index.php/Dev:Ref/Supported_platforms they do have developers for Mac OS X 10.5, but ever since 2.64, they’re not releasing any more official builds. I could get some from http://builder.blender.org/download/ , but so far they all had no Cycles script, and at least the one I tried had a serious bug in the VSE, which made it completely unusable.
So, it clearly can be built on OS X 10.5, but they refuse to release it. Anybody know why? And anybody know where I can get it anyway, preferably with Cycles?