Read here first: Inside Adsense blog Google help: Expandable Ads FAQ "Introducing expandable ads on Adsense sites" Basically it's like link units, but in your regular boxes. People click on the ad and it shows them an even bigger ad. But you're only paid if they click on the expanded version to visit the website.
I do not think that this is available to all. Rite now this is on the will of advertisers of USA and whose sites they will choose. Seems to me that Google is now doing too many experiments to beat this slow down. Good for us