With the iPad, Apple introduced a new user interface element called the popover. This was mainly used in apps where there was a navigation list and a larger view showing more information: what Apple calls a Master-Detail application.
In landscape mode, the list is shown fully on the left of the screen and the details on the right. Switch to portrait mode and the list disappears, but is accessible from a button in the top left that shows the popover.
This is a very useful design and is what I used in Pic-a-POD for iOS: the list shows the available PODs, with a thumbnail and caption for each. Tap on one of these and the larger picture is downloaded and displayed, along with further info and more options.
However Apple has quietly changed the way popovers work in iOS 5.1 and I don’t like it as much. Instead of dropping down with an arrow pointing to the button that triggered it, the popover now slides in from the left, taking up the entire height of the screen.
There seems to be no way to fix this. I have filed a bug report with Apple, but if this is a deliberate design change, then I expect it to stay like this. The best solution would be if a popover could be configured to use either method.
Here are screenshots from the iPad Simulator running in 5.0 mode on the left and 5.1 mode on the right, which show the difference once the popover has appeared: