That is a Nook ebook, open in my web browser on the iPad. According to the B&N website, that is not supposed to be possible. But there you have it.
I still have no clue when this support was added, but I do know that iPhone was not similarly blessed. According to a friend on Twitter, Nook for Web won't work on the iPhone. That would be regrettable except that Nook for Web doesn't work so well on the iPad either.
I've been playing with the app and noticed that it still has a number of rough edges. The iPad support is drastically incomplete, with N4W missing a number of key features. While this app does have some basic formatting options, it is missing any annotation options. There's also no full screen mode (this is available when using N4W on other platforms), and the app was unable to remember to keep me logged in to my account.
N4W is currently unable to sync my reading position with my account, dashing any hope of using N4W in combination with other Nook apps. N4W is also missing a mobile-optimized library page; instead you are expected to use the regular B&N website (yech). And most importantly Nook for Web was unable to read over half of the ebooks in my account, and it was completely unable to download content so it could be read offline.
I would really have liked to have written nice things about Nook for Web, but so far as I can tell over the past 11 months it has only been the subject of token efforts to add more features. This app is embarrassingly limited when compared to its competition, Kobo Instant Reader and Kindle Cloud Reader. Both of those can be described as finished and polished; the same cannot be said for Nook for Web.
Fortunately readers don't have to depend upon this app. B&N is still actively developing the Nook apps for iPad and iPhone, with the most recent update having been released in March 2013.
With that in mind it is my guess that this limited support was a side project that added unofficial support for the iPad.