Since yesterday i received three reports of people seeing screenshots of the sites in normal serps (like in WMT). They were at the left of the normal listing. Didn't see them myself though.
6:02 pm on Oct 3, 2011 (gmt 0)
Today I'm seeing a three column layout. Left column is search options, center is the the serps, right is adwords ads.
When the cursor hovers over a result a box with two right pointing arrows appears. If the cursor hovers over the box, adwords is replaced by a large site preview. Scrolling down the results with the cursor in line with where the boxes appear keeps the previews active, adwords does not reappear until the cursor is moved from that line.
6:18 pm on Oct 3, 2011 (gmt 0)
Yeh, that feature is rather new, but well known already, but the screenshot wich i saw of what i described in the OP, was rather astonishing.... Probably G. testing on a small scale...
9:13 pm on Oct 3, 2011 (gmt 0)
Did the three people have something in common? Geographical area, browser type? I'm still getting the same thing that google rolled out, what, a few weeks ago? Or possibly months. I tend to misplace time. The version with the pale blue hover-arrow that brings up a Preview. Or, in the case of my newer pages, No Preview Available.* (As a curious aside: in Safari they listed two versions of the same page-- that is, two URLs-- reflecting a brand-new redirect. Camino only shows the new version.)
Incidentally, the SERP screen looks really nice if you have a powerful ad blocker, because the area along the right is completely empty. Didn't even realize there was supposed to be something there until I tried a different browser. I thought they were keeping it free for the Previews. Ha, ha. As if.
* Quick detour to Error Log confirms that they tried to pick up a Preview-- but only once. They didn't put in a fresh request for each new SERP. So the Google Web Preview robot has some type of robotic intelligence that ordinary robots haven't got.