This is a suggestion for key mapping when using a physical controller. For example a device like the ipega PG-9023 extending Bluetooth gamepad, or the built-in physical controls of an Archos GamePad, Wikipad etc.
When the user plays in portrait orientation, since physical controls rotate with the device, the desired key mapping to use will change. Of course the physical button positions in portrait mode aren't ideal (D-pad below screen, action buttons above), but when mapped differently they should still be very usable.
For example, with the D-pad functioning normally in landscape, if the user rotates the device 90 degrees anticlockwise then the new D-pad mapping would be: down -> right, up -> left, left -> down, right -> up. And you'd probably want A/B/L/R mappings to change too.
So my suggestion is to allow different key mappings to be set for different orientations, and automatically use the correct mapping when the device orientation changes.