It's not about read/unread but rather visited/unvisited. Specifically the following CSS in http://www.forum.nokia.com/css/ui.css
specifies styles for all anchor (link) elements and a specialization for hovered-upon lnks. However, there's no specialization for the a:visited property and therefore URLs already visited appear using the same style as any other link.
This was already an issue with the previous version of FN and I did mention it in some feedback post back then. The version before that had blue color for unvisited and purple for visited links.