This is a nice idea, but I agree with bfarber. TSP has a nice idea with mirroring, similarly to moving with link, but there are issues with that as well. The original idea is hard because to achieve well because the forum and topic tables would switch from a one-many relationship to a many-many relationship. This requires a new table and it will add a lot of DB overhead (LEFT JOIN) to each query for a topic. Plus you have to add forum_id references into each URL in order to provide proper navigation. There's a lot other reasons that make this difficult and inefficient.
TSP's suggestion is what I might go for. Last Post, replies, and view counts could be a gray area depending on permissions. It could be accomplished with the hooks system I suspect.
Keith
