On the iPhone or iPod touch, a modal view controller takes over the entire screen. This is the default
behavior and the only possibility on these devices. On the iPad, you have two additional options: a
form sheet style and a page sheet style. You can change the presentation of the modal view controller
by setting its modalPresentationStyle property to a pre-defined constant –
UIModalPresentationFormSheet or UIModalPresentationPageSheet.
The form sheet style shows the modal view controller’s view in a rectangle in the center of the iPad’s
screen and dims out the presenting view controller’s view.
The page sheet style is the same as the default full-screen style in portrait mode. In landscape mode, it
keeps its width the same as in portrait mode and dims the left and right edges of the presenting view
controller’s view that stick out behind it.
原文：http://www.cnblogs.com/playerken/p/4167057.html