Ive noticed that on the forums plugin whatever the content of the first post is, is also copied as the <meta description""> for that page.
for e.g. a first post that says
" Below are some of the most frequently asked questions at myteenblog.com"
<title>Frequently Asked Questions.</title><meta name="keywords" content="teen blogging, teen blogs, teen advice, teen chat" /><meta name="description" content="Below are some of the most frequently asked questions at myteenblog.com" /><meta http-equiv="Content-type" content="text/html; charset=UTF-8" />