Yes there are quite a few blogs on this but none of them is as complete as i would like it to be. So i am documenting my experience here.
1) First download the bits from Oracle. Key is to download the "Generic" Package Installer from here
2) Before starting to install you have to trick the installer into thinking that the local JDK is the generic Sun JDK. If you skip this step the installer will not accept the default Mac OS X JDK and complain that it is Invalid.
$ cd /System/Library/Frameworks/JavaVM.framework/Versions/1.6.0/Home
$ sudo mkdir -p jre/lib
$ cd jre/lib
$ sudo touch rt.jar
$ sudo touch core.jar
3) Now install WLS using the following command. The installation is pretty straightforward.
$ java -Xmx1024m -Dos.name=unix -jar wls1031_generic.jar
4) The next challange is to overcome the java.lang.OutOfMemory error that occurs when you try to access the console at http://localhost:7001/console. As a result the server hangs.
To recover from this error you actually have to kill the JVM. So edit the user_projects/domains/mydomain/bin/setDomainEnv.sh script first and change the line "if [ "${JAVA_VENDOR}" = "Unknown" ] ; then" to "if [ "${JAVA_VENDOR}" = "Sun" ] ; then"
5) One last thing that is recommended is to set USER_MEM_ARGS="-Xms256m -Xmx512m -XX:MaxPermSize=128m" in startWebLogic.sh script. I added this as the first line.
6) Finally start the server
$ cd user_projects/domains/mydomain && ./startWebLogic.sh