We have figured out why MacPyMOL was crashing for a select subset of users on start-up. It only happens if you extract our ".tar.gz" archive using StuffIt Extractor instead of Mac OS X's built-in extraction. The ultimate cause is rather esoteric: I had inadvertently enabled the Hide File Extension attribute on the splash screen PNG file, and StuffIt Expandor seems to have a serious bug extracting such files.
MacPyMOL should just work if you use Safari's default ".tar.gz" handling when downloading from http://delsci.com/beta. With Firefox, you need to make sure that StuffIt Expander is not used for ".tar.gz" files.
If all else fails, simply pop up a terminal and resort to:
tar -zxvf macpymol-0_99beta29.tar.gz
PS. Special thanks to Prof. James Berger at UC Berkeley for lending us use of his laptop for a few hours to help us to figure this all out.