Single registration for multiple forums

We have a site which supports many communities each of which with their own forum (http://dmz-gaming.com).

I would like to have a single logon and profile for all the forums but maintain control over permissions locally to each forum (when I refer to 'forum' here I mean installation of punbb, seperate location, files and databases).

As far as I can see there would be two ways of doing this:
1, have a single database of all members details that are not pertinant to permissions and direct all the forums to refer to this.
Pros
Scalability - as more forums are added the code of each will need tweaking and the user table will grow but the processing of each new registration or profile change will be constant.
cons
Means devorcing the basic registration details from the permissions and may require table changes
Harder build

2, Modify the registration and profile change code to apply changes to the databases of all the forums
Pros
Table structure can be maintained
cons
Poor scalability - as each forum is added the processing for registration and profile changes becomes more complex and processor expensive

Does anyone have any better ideas or even a solution already that would offer this function.

I have considered subforums but can't find code to try it out as this link seems to have died (retrieved from wiki and search)
http://punbb.org/forums/viewtopic.php?id=8381