Here's what one looks like, for the curious:
Mobile: http://i.imgur.com/kQlmh.png
Desktop: http://i.imgur.com/ZiOqX.png
This appeared in my news feed last night, and is still there now. It moves like a normal item (i.e., its not 'sitckied' to the top or anything like that).
On mobile its pretty jarring. Bigger than any other posts.