These are the screenshots of the latest Pandora iPhone app. In the latest release, it introduced a draggable area on the bottom bar. When you drag it up, it exposes the sound volume control.
I think iOS has first introduced this draggable bar concept in a couple of places.
Shown in the above is the lock screen; when you drag the camera icon up, you get straight into the camera app without having to enter your passcode, which is really handy when you are trying to snap a photo of a precious moment.
Another place I’ve seen this drag gesture is on the notification center.
I think the draggable bar UI metaphor is intuitive, and it is easier to use than the tap gesture. Whenever you are designing a mobile user interface where something slides in and out, you should consider using this metaphor.