I am very happy with my decision to go ahead and create the cms before the forum itself so far. One big reason for this is that it will make it much quicker and easier to handle the social network versions of the forum because they can just be new pages with different templates on them.
Although I could have quite done this easily using templates in Django, being able to decide what modules to use via an admin interface will make changes and updates much easier. Especially when I make introduce the ability for people to host their own forums. Obviously this affects the way that I develop the forum so that functionality is broken into individual modules as is practical to allow customisation.
This approach also allows for more customisations, such as having different menu modules that people can use for their forums etc.