It seems that couple of weeks ago:
Google has modified it’s robots.txt file to include a line which may signal the beginning of a new search feature — similar content search. This innocent looking piece of text (“Disallow: /relcontent”) appears related to “Disallow: /relpage/” which has existed in the file for a very long time.
Zdnet Blog also wrote at that time:
It’s unclear how this speculated feature could differ from the existing “similar pages” functionality – I thought using “related:” was designed to find similar content already.
Well after all it seems that things started to get clear. It seems that all is about some snippets to get inserted and syndicated by users in their webpages that look like this one:
The feature seems to be still in tests, but is probably yet to come out soon.