It looks like Google has just started adding additional information to results from forums in the SERPs. Notice right under the title below where it says:
5 posts - 3 authors - Last post: Aug 21
I've seen it for forums build with different forum software including vBulletin and SMF, though not PHPbb or punBB.
You can see it in Google's SERPs here.
EDIT: someone on Sphinn pointed out that Graywolf mentioned this already on his blog.