I'm not familiar with Sun, but IBM has two, one for portal administration, and one for portal development. IBM Tests 399 and 829 I believe.
The administration one is tough, and you need lots of IBM WebSphere Portal hands-on to pass it. The development one is very focussed on JSR168, so if you know development, you'll do good.
My JSR168 Portlet Development Book started out as an IBM certification guide, but I took the IBM specific stuff out and made it more about portlet development in general. Still, it's a solid certification guide for the IBM exam as well.
I've got some free mock exam simulation tests on my website. Feel free to test your portlet developer wits: