Set up a blog on your website which discusses content related to your website and then have the blog feedback mechanism set up as an open forum in your main forum. Obviously this only works if you have the time to stay on top of moderation.
We have a website which is paid subscription based. The discussion forum is divided into two areas, one which only subscribers can view/post(ex: a place to discuss content they have payed to access), and a second general area where guests can view but not post. The drawback to this is that the membership and thus the discussion is completely subscriber based, so the board grows slower than a free registration board would.
The idea with adding the 'open' blog discussion is to get visitors to interact and thus increase their likelihood of subscribing.
That's a really interesting idea, Reflection, for a couple of reasons. First, blog feedback mechanisms tend to be rather crude - they tend to be single, linear threads with minimal, if any, user authentication. Forum software gives you far more control over who's posting (and dealing with problem users & spammers), plus it can allow for more complex conversations.
Second, as you point out, if you can get people into your forum there's a chance they will stick around and become paying members.
I've seen forum software used this way, but probably to much lesser effect - the owner/admin can post "announcement" threads, and then link to discussion threads where all members can participate. This is fine for experienced forum users, but a first-time visitor would probably be more likely to read a blog-type article vs. deciphering the forum/thread structure.
Let us know how this works out. Also, I'd encourage you to share your experience so far in the Paid Forum Membership [webmasterworld.com] thread.