It is a thought. If there is an API to the cache plugin then it could, indeed, be flushed. Although there would be a small penalty to pay for doing that and the sense in doing it at all would largely depend on how busy your forum is.

There are, however, other issues with caching the forum page. The time since a post was made becomes quickly inaccurate. Whether a user is logged in or not can be displayed incorrectly. In fact they could be served the wrong forum data either way which, for a security conscious site with public and private forums would be a disaster.

i would worry that having to flush a cache each time someone logs in or logs out; when a new post is made, when a new user registers etc., etc., might just end up not only defeating the object but costing time. But I will think on it.