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At this point I'm not aware of a good step-by-step tutorial for Spring Portlet MVC. I suggest you follow the tutorial for Spring Web MVC in order to learn the general framework and then you can look at the sample applications for Spring Portlet MVC to see the unique features present there. You can find one in the CVS source code for Spring and another on the Spring Portlet MVC wiki space.
You should also take a look at this FAQ entry for some good introductory links.