I found this blog entry that details how to get Weblogic 10.3 installed on OS X. I don't know if something has changed with Snow Leopard, but I found that a few things didn't work correctly.
Below are the modified steps I took to get it installed.
From that directory, enter the following commands:
sudo mkdir jre sudo mkdir jre/lib sudo touch jre/lib/core.jar sudo touch jre/lib/rt.jar
Execute the following command from the directory you saved the installer into:
java -Xmx1024m -Dos.name=unix -jar server103_generic.jar
In the installer, choose to create a new BEA Home: /Users/dan/Applications/bea (substitute dan with your user name)
Select custom install type and uncheck the Workshop option (and Weblogic Web Server Plugins if you don't need them).
The installer should detect Unknown 0.0.0 as a Local JDK. Click Next.
Keep clicking Next and let the installer run to completion.
When the installer completes, use the /Users/dan/Applications/bea/wlserver_10.3/common/bin/config.sh script to create a new domain (without clustering and/or RDMS capabilities).
When that completes, go into your new domain directory, edit the bin/startDomainEnv.sh file and change the following line:
MEM_ARGS="-Xms256m -Xmx512m"
to:
MEM_ARGS="-Xms256m -Xmx512m -XX:MaxPermSize=128m"
You should now be able to run the startWeblogic.sh script and start the server.