Blank screen on iOS coming back from pause

With the latest patch (5.1.1p2) we have started seeing an issue on iOS devices where the screen will be blank if resuming from a paused state. A user will be playing our game, set it down, the phone will go to sleep and when they turn it back on the screen will be dark - missing all 2D game assets. The only thing visible is a 3D particle effect. Our game is primarily 2D. Another odd thing is that, judging by the position of the particle effect, the game is in landscape mode - which is odd because we have set our game to lock into portrait mode.

In summation, resuming the game by waking the phone will show our game with no 2D assets visible, no actions possible, 1 3D particle effect still showing on an otherwise dark screen, and stuck in landscaped mode which it shouldn't be allowed to be in.

Any help would be appreciated!

Thanks, Mike