Google is once again testing placing dividers between search results. This new test was spotted by Vlad Rappoport and posted on Twitter. Google has tested dividers in the search results before but they really have not stuck.
Here is the screen shot from Vlad:
Google is always testing things with the user interface, I've covered well over a thousand of these tests. You can browse them all in the Google user interface category if you have time.
Forum discussion at Twitter.
