Apple recommends that cancel buttons not be shown for action sheets on iPad:
Because taps outside the popover dismiss the action sheet without selecting an item, this results in a default way to cancel the sheet. Including a cancel button would therefore only cause confusion.
But the guidelines do allow exceptions to this rule:
However, if you have an existing popover and are displaying an action sheet on top of other content using an animation, a cancel button is still appropriate.
So you would think following this guideline is as simple as not specifying a cancel button in most cases, and only specifying a cancel button when you have carefully considered the exception case, right? Wrong. Even when you do specify a cancel button, it looks like the iPad runtime automatically always hides it.
So how do you provide a cancel button in the exception case? You could just provide another normal button that has cancel behavior, but that's not ideal because you really want the difference in appearance that UIActionSheets on iPhone provide by default. After some tinkering, the best solution I could find was to set the actionSheetStyle property of the UIActionSheet to UIActionSheetStyleBlackOpaque.
I can speculate why this works, but I don't know for certain so I'd rather not. If you know of a better/more correct way to do this, please do leave a comment.
I am surprised that Apple was mindful enough to list the exception case, and yet careless enough to not provide documentation on how to handle this case - they even seem to have forgotten to document that the iPad automatically overrides user-provided cancel buttons by default.