I should clarify. By "association" I meant being owned/operated by the same people.

As for bringing them together under a single web ring, it's not something I would want to rule out, but I'm not sure of the advantages. I guess it all depends on how you go about implementing it. I have a feeling it would end up either as a few links on the bottom of a page, or some sort of tabbed virtual sections on a site that ends up reducing a site's character. In my opinion it would just be better to have members of each forum mention other sites if they have found them useful.