A Crea8site Forum thread named MSN trying a different SERP reports that there is something really called MSN Search Preview. In the past, MSN Search Preview referred to the "Technology Preview" released at http://techpreview.search.msn.com/ when they first began beta testing the new MSN search technology. But this MSN Search Preview looks to be totally different.
In the thread, Barry Welford identifies referral data from http://sea.search.msn.co.uk/. I looked back at my notes and found that there was an archive here that discussed the URL sea.search.msn.com, under the title of See the Old MSN Search Results, found over at SEO Chat. But this is different or maybe really old, I am not sure.
If you go to the search query listed by Barry Welford, http://sea.search.msn.co.uk/preview.aspx?&q=ten+commandments+of+communication, you will notice it shows little snapshots of Web sites from the sponsored results. Towards the bottom of the page I saw a link named Website owners: prevent your page from being previewed. Officially this looks to be called, "Search Preview image" and there are tons of more details at MSN Help. There is a page specifically about this here, that says:
Search Preview displays small images of websites in your search results. These are useful in deciding if you want to visit a site. Small preview images are displayed for the first six search results on each results page. To view Search Preview images, you must: Use Microsoft Internet Explorer as your web browser. Set MSN Search as the default search provider for the Search Explorer bar. For more information, see Search Assistant uses a search engine other than MSN Search. Turn on the Search Explorer bar in Internet Explorer. For more information, see Open the Search Explorer bar. Use the Search Assistant in the Search Explorer bar. For more information, see About the Search Explorer bar.