deadsea - 5:01 pm on Jan 9, 2013 (gmt 0)
For blogs, Google is simply looking for "posted on <datetime>" multiple times in the text of the page. I know this because one of my clients was a jobs board that had listed the date and time that each job was posted on the site. Their site appeared in the SERPs as if it were a blog (and included in blog search.)
I'm not sure exactly what triggers the forum recognition, but its got to be something similar.