In November 2010, Google launched instant previews for the search results. The concept behind this feature was not new and many called Google a copycat for launching the feature.
Either way, the feature has stuck, and it still works in the search results.
Google has recently been testing these instant previews in the AdWords ads. Matt McGee reported at Search Engine Land that these instant previews give searchers a chance to view the ad's landing page prior to clicking on the ad.
It is unclear if and when Google will launch it.
I have to assume that clicking on the preview feature does not cost the advertiser any money.
Here is a picture of this in action by @ZeonSearch:
Personally, I'd like to see this feature added to the ads.
Forum discussion at Sphinn.
This post was written earlier and scheduled to be posted today.
Update & Official: This is now official:
Now, we're bringing the same benefit to ads with Instant Previews for Ads. Starting today, the Instant Previews icon will appear next to ads on Google.com allowing users to preview the ad's landing page. With Instant Previews, your customers are able to quickly preview a page to see if its content matches what they're searching for.
By allowing potential customers to preview your site before they arrive, Instant Previews helps you get even more highly-qualified traffic to your site. Even better, Instant Preview clicks are free of charge -- you're only charged if a user clicks through to your actual landing page.
More details at this help document.