A WebmasterWorld thread shows that dozens of people are reporting significant drops in search referrals from Google's Image search product over the weekend. What is very interesting to note is that no one is reporting increases in traffic, just wide spread drops in search referrals from the image search vertical.
This can mean two things, that I can think of right now.
(1) The image index has been updated and dropped many images it once had (2) The vertical integration of Google Images on Google Web Search has been tweaked enough where Google is now showing less images in the One Box results in Web search results.
That is what I got so far.
Forum discussion at WebmasterWorld.