A WebmasterWorld thread has one SEO noticing that Google is testing a new search results user interface. The new interface tends to highlight the background of a single search results, in a light blue, when a mouse moves over that result.
Supposedly, it looks a lot like the Google page previews test, but without the magnifying glass and page preview option. So I suspect it looks like this (I mocked this up off the page preview screens):
Forum discussion at WebmasterWorld.