If you put the post into folder posts and portfolio, jekyll will naturally defined two categories
root
|-- posts
| `-- _posts
| |-- 2012-01-12-post.textile
| `-- 2012-03-22-post.textile
`-- portfolio
`-- _posts
|-- 2011-04-05-post.textile
`-- 2012-02-02-post.textile
Then you can have a generator that will create the category page, you may then take inspiration from this post: http://www.justkez.com/generating-a-tag-cloud-in-jekyll/ except that instead of generating the tag page, you generate the category page.
EDIT
There is also this plugins generate_category.rb that can help you: http://recursive-design.com/projects/jekyll-plugins/