Just to add my 2 cents to this discussion:
This is not really new. I support many forum sites and the tags like "Last post: May 3, 2009" have been appearing for more than a year in most SERPs I'm looking at.
However,
#1 - a fresher result DOES NOT guarantee a higher spot in SERPs
#2 - the date is
ALWAYS, 100% of the time wrong
It bothered me in the beginning - I tried to ascertain how in the world they come up with the date that's always wrong - but nothing conclusive came out and I just learned to ignore it (possibly at my own peril).
I can't tell you how they do come up with the date but, based on what I looked at, I can list what they DO NOT use for the calculation.
They DO NOT use
- HTTP headers
- Dates mentioned in the text of the page
- if there is an RSS feed listing the page, they do not use <pubDate> field
- if there is a sitemap, they do not use <lastmod> field
I guess, I'll have to re-phrase the above: if they use the data, they come up with the date that
does not equal to any of the above mentioned dates. In many cases they guess even as much as the year wrong.
I could not see a clear indication that a fresher page based on the "Last post: #*$!" tag means higher SERPs. That was a year ago. If it changed since, I'd be interested to hear about it of course, but I have to tell you: it will be really hard to game this because you never know how they come up with the date and why it is always wrong.