“Keystoning” refers to the distortion that occurs when the camera sensor is not in the same plane alignment with the subject and is very noticeable when, say, pointing your camera upwards at a building or big ship.
According to this rumor, a firmware update to the Olympus E-M1 will provide a software feature in camera to correct for this distortion: (FT4) E-M1 firmware update will add “keystoning” feature. | 43 Rumors.
Keystoning also occurs when shooting 3D and using toe in on the left and right cameras to position the primary subject at the viewing plane. Some 3D photographers choose to shoot 3D using cameras that are strictly parallel in their forward view, while others toe in the camera slightly (I believe director James Cameron does this in his films). The non-parallel view is helpful for alignment but can create keystone effects in the image. These keystone effects can be corrected during editing (e.g. Sony Vegas and Movie Studio include features for this) by warping the original (but keystone distorted) image back to normal.
Which leads to a question: would this firmware upgrade enable toe-in of a paired E-M1 cameras with in-camera keystone conversion for 3D application?