Patrick Altoft spotted a real winner, and at first I thought he had some weird browser extension, but he did not. Google is indeed testing page previews with a magnifying glass.
Here is a picture from Patrick:
Other people in the UK are seeing it, I cannot replicate it.
In June 2004, Ask Jeeves went with binoculars on the SERPs, which is pretty much a similar concept. Google reportedly tested this concept in August 2004 (or it may have been an extension). But Google did give searchers an option for page previews in November.
Google did confirm this is indeed a real test and not some sort of browser extension.
For more information on this, check out the Techmeme links.