If you are planning to do clustering in weblogic 11g with SOA, I can suggest you to do active-active clustering for soa components and Active-passive clustering for Weblogic admin server. The reason behind it:
* Application configured in SOA will be avaliable to end user on HA mode. If admin server goes down then there is no impact on application access.
* You can manually swich over admin server to seond node and start it there without impacting soa managed server.
For implementation please see below link.