While reading a slideshow on How People Really Use the iPhone. It seemed a lot of users are confused when looking for a "zoom" icon; finding the magnifying glass to mean "search" and the plus sign to mean "Add", while both have somewhat classically been used as zoom symbols.
Perhaps important to note; there very rarely is a zoom button in iPhone/smartphone apps because the pinch-zoom gesture is usually used where applicable, but this is not always the case. This does mean that some of the user's confusion was due to the zoom button not existing.
My experience with Android and Web Apps also leads me to believe the magnifying glass and Plus are very commonly used to mean Search and Add respectively.
So what visual metaphor or icon can uniquely and intuitively suggest "Zoom" rather than Add or Search?