The original Smartlabels consisted of Bulk, Notification or Forum. Only Forum is in the new categories. As I've written elsewhere, all these labels and categories are confusing and redundant.
Regarding the Smartlabels lab, I've had that enabled for a long time but don't remember how it works. I reviewed in Labs that these are labeled automatically, but how did Smartlabels know to use Notification rather than something else? You wrote, Marko: "When I turn on the lab, Gmail prompts me to select a category for your post, and has 'Forums' selected." I don't recall this procedure at all. I guess I don't need to review it unless I decide not to use the new categories in the Inbox.
Finally, the Smartlabels lab says Report mis-categorized email from the 'Reply' dropdown menu. At the moment, I can't find this menu. Maybe it's time for bed.