First, Celestia's homepage does not give the most current version of Celestia for Mac OS X -- only version 1.3.1 is listed, when in fact version 1.3.2 is already available on their SourceForge files page. Second, the install process is a bit clumsy (though correctly documented): you need to install the application in /Applications, and a whole lot of other files in /Library/Application Support.
Finally, it looks like there's a bug in Apple's implementation of the OpenGL GL_ARB_vertex_program, which hits Celestia users on some machines (mine is an iBook G4). The bug is nasty: when Celestia tries to render a planet, the Mac suddenly crashes, and everything is stopped. To bypass this bug, it is enough to edit the celestia.cfg file (in the /Library -> Application Support folder), and uncomment the following line (by removing the #):
# IgnoreGLExtensions [ "GL_ARB_vertex_program" ]
[robg adds: I had just recently installed Celestia for consideration as a future PotW. It is a bit clumsy of an install, but you can put the app wherever you like; I have it running on my Apps partition, and I have the resources in my user's Library -> Application Support folder, not the global /Library. If you haven't played with it, it is an amazing app, and well worth a look.]

