How to sort users in a view by the number of nodes authored of a specific node type? I was happy to find that in Views 3 I can sort users in a view by the number of nodes they have authored (Using a relationship "User: Content authored" and the Aggregation / 'Group by' - Count function, see here).

How can I do the same, but counting only nodes of a certain node type? (I'm actually using this in a view of profiles (Profile2), but using the relation "Profile: User UID" the solution would probably be the same).
Not sure if I understand you correct, but have you tried setting up a normal filter:

Content: Type (= nodetype)