I don’t know how long this has been around, but I only noticed it late last week. Google has added a bit of metadata to their search results when a result comes from a forum (or a site structured similarly). Check out the results from this search:

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Notice the number of posts under each link? Google has detected that Stack Overflow is a “post-based” site, like a forum, and is giving us that extra bit of detail in the search results. For some forums it’s even including the date (and sometimes time) of the last post, which is really handy for determining how up-to-date the conversation is.

Update! For the propeller-heads among you, it looks like this is the RFC that governs threading in published feeds (which is probably how Google's doing it).