There is an app for this. It is called "Camera" and it is preinstalled on your iPhone.
Simply place a facetime call and then grab another iPhone, point it at your facetime screen and use the camera feature to record it.
whats with the smartass replies?? legit question.
http://mac.appstorm.net/roundups/utilities-roundups/10-screen-recording-tools-for-mac/
Looking at the same myself - anyone used any of these?