A good portion of the IBM Test 399 and Portal test 829 has to do with development. All of the correct development answers have to do with JSR168, although I did see some references to the JetSpeed PortletData as some incorrect options.
Some of the extra WebSphere Portal Server specific stuff has to do with skins and themes, namely the various wps custom tags you can use in a skin.
There were also a few questions on the IBM
Struts Portlet, which is a little different from the Apache Struts Portlet Bridge.
There is probably one question on the WebSphere Portlet Catalog, a question or two on the personalization engine, and a little on WebSphere configuration, where you need to know a little about XMLAccess exports.
So yes, to really kill the certification, you need to know WebSphere Portal Server (WPS), but having said that, developing on JetSpeed2 and learning the JSR-168 API is not a bad way to start.
Good luck!
-Cameron McKenzie