The following reference I have used for the exam...
1. IBM Rational Application Developer V6 Portlet Application Development and Portal Tools (Redbook) (sg246681). 2. JSR 168 Spec from sun. 3. http://www.portorials.com/ (here you could find some CBT).it helps how to develop the portlets in RAD.
Most of questions based on JSR 168 portlet .. no need to more concenrate on IBM portlets. just you have to know the difference.
have a nice,
Cameron Wallace McKenzie
author and cow tipper