I think I saw something similar a while ago. Was probably even discussed here.
Consensus was (if I remember correctly) that Google is running beta tests on new ad formats at all times. This might be such a situation.
The only question I am wondering about - when the user clicks "show more", I assume that more ads will be shown. Will the publisher be fully credited when the user clicks on an ad on that page? Or where will the money go?
Also, one might argue that we should be credited once the user clicks the "show more" link. After all, the user leaves our site to see a parked page (stuffed with ads)...
I saw this the other day on my site. I also remember the discussions a few months ago when it first appeared on some sites. Perhaps mistakenly, I thought that when a user clicks that link they don't leave the site. Instead, I'm under the impression that the ads change on that same page.