Four years ago, Google introduced expandable AdSense ads but truth is, I've never seen an AdSense unit that expanded in all the four years since the introduction.
That seems to be changing, where one publisher noticed an expandable ad unit on his site just recently. He posted about it on WebmasterWorld saying:
Wow, what an ad, I saw it on one of my sites today and it's fantastic, passing my my cursor over the ad and a box in the bottom left read "expanding" and up it popped into a 900 x 400.
Here is a picture of the expanded ad format, that only expands on click:
Have you seen this ad format before?
Forum discussion at WebmasterWorld.