$ java -Dos.name=unix -jar server900_generic.jar
When you press return, the graphical installer will run and install WLS 9.0. Native libraries that provide a performance boost will not be used on Mac OS X, but this will allow the server to be installed and start up. I've posted a (slightly more detailed) write-up on my own blog.
Disclaimer: I work for BEA Systems (Federal), and they do not support WebLogic Server on Mac OS X.

