I am not a 100% sure if this is new, I know Google has been testing many variations of new previews – but @SEMantiks tweeted that Sitelinks now have their own Instant Previews.

Yes, when you go to a sitelink, and hover over it, a preview may show up.

Here is a screen shot, click on it to enlarge:

Is this new?

There is some new discussion around preview tests by Google at UK Business Forums.

Forum discussion at UK Business Forums.