Hey, what a cool idea! Thanks for sharing it with us.

I'll cast my personal vote for setting up a blog rather than a forum, mostly because it's really easy (and free) to do. It probably makes the most sense to make use of one of the online blogging services and just add it as an external link to your library website. WJ has some 'get started' info in the [url http://webjunction.org/do/DisplayContent?id=12354 ]Easy Steps to Creating a Blog[/url] article and in the [url http://webjunction.org/do/DisplayContent?id=12146 ]Start a Blog in Five Minutes[/url] flash tutorial.
Encouraging your members to actually post comments in either venue is the next challenge. IMO, that's best accomplished with a face-to-face workshop to help overcome those initial posting fears.
I'd love to hear from others if anyone has tried this. I know there are some wonderfully active Friends' groups out there.