My first idea is that the portlet framework is already MVC compliant by itself (i.e clean separation of action/render).
Then for a more elaborated framework there are some of them that offers to run in a portlet environement : webwork2,tapestry,JSF
The best option is probably JSF if you like component based development. It is hard to learn though. Otherwise the others seems to be quite classic with respect to be a struts like action based framework.
I would recommand to avoid the classic struts at all cost.
Regarding other design patterns, I am thinking about some but I had no time to formalize them yet. Perhaps some other people have discovered DP while doing their dev, that would be interesting to hear about that.