It must be regional. Here in Florida we experienced a sneak-peak of Google's so-called Caffeine 2 days ago. Google's homepage and look and feel of the search results changed, as well as the positioning for a lot of sites. The change was, in my opinion, awesome and incredible. The changes, however, were short lived and Google's old homepage and old results are now back in effect. *sigh*
SE's changes happened periodically like PR otherwise the process happens all the time.....As a lots of websites doing their promotion every time so changes are quite possible.