A user on the forums asked how to build a tabbed category list. Following the principle of "there's more than one way to do it" I answered with a few different approaches how to solve that within Serendipity.
Let me say, that it can be achieved easily with little effort, thanks to the Smarty templating and the plugin API. You can even display a tabbed output of staticpages (see another forum post), since they assign $staticpage_XXX variables to the Smarty scope which you can check on in your index.tpl template. As well, building a function to fetch a list of staticpages (like the sidebar plugin for the staticpages does) could be integrated to not have any static HTML waste your templates. :)
Now, for the final question: Who is willing to contribute a tab-based category template, now that everything can be done in a template without modifying any core code or even requiring a plugin? :-)