A smartphone today has two faces, and by smartphone I mean a proper iPhone or Android that has an accelerometer included in it. Anything without that is probably not considered smart enough. While one is designing for these devices, one should keep in mind that there is always that possibility to include an experience when the phone is tilted by detecting its orientation. A classic case for this is the calendar app on iPhone. You can tilt the phone to see a weekly view from any point on the calendar view, and it makes sense since fitting the weekly view in portrait mode will be completely useless!
Here’s a screen shot.