This is not the first time I have gone looking for this patch, which allows you to install Oracle SOA Suite 10g with an Oracle 11g database, and I know it wont be the last. So here is the link:
This will ask you for your Metalink credentials. Alternatively, you can just log on to metalink.oracle.com and then search for patch number 6265268.
You may have to edit the irca.bat script to update the name of the jdbc library to ojdbc5.jar, instead of ojdbc14.jar. IRCA will ask for a Java 1.4/5 JDK. There is usually one installed with the 11g database, in the jdk directory in the database home.