At Alex Olwal’s web page you can watch video and download a CHI 2008 paper describing research on rubbing as a gesture for zooming. Rubbing up and to the right zooms in, up and to the left zooms out.
Their evaluation showed that it was better than other single-finger methods for zooming, though fatigue can be a problem if the screen surface isn’t smooth.
They also tested two variations on combined two-handed tapping and zooming techniques (e.g. one hand points or zooms and the other can tap to select).
Their focus was on touchscreens that report only a single finger for applications like public kiosks.
The paper is "Rubbing and Tapping for Precise and Rapid Selection on Touch-Screen Displays" by Alex Olwal, Steven Feiner, and Sanna Heyman.