Positional feedback

http://www.designworldonline.com/uploads/ImageGallery/avago%20aedt-9140-1.jpg

Positional feedback is widely used in RC servo's, scanners and printers. And actually the good old PC ball-mouse used a similair technique to detect the directions. Since DC motors are less expensive then steppermotors and with the correct control system they also consume less power.

It all depends on the application and the type of motor + controllers.

If this technology will become popular for 3d printing is hard to say, all it takes is a cheap motor that's easy to get hold of, and a design that's equally cheap to implement and easy to understand.

Here are a few options to look at:

http://www.yeggi.com/q/optical+rotary+encoder+discs/

http://www.infineon.com/dgdl/Position_Feedback_for_Motor_Control_Rev.1.1%5B1%5D.pdf?folderId=db3a30431f848401011facc1c83b4674&fileId=db3a3043293a15c4012940fa76d32a0b

http://www.ikalogic.com/closed-loop-speed-and-position-control-of-dc-motors/

http://www.citizen-micro.com/pro/encoder.php

http://www.anaheimautomation.com/manuals/forms/encoder-guide.php
single-ended - inside

http://madpenguin.ca/blog/2011/06/14/tutorial-use-an-old-inkjet-printer-to-learn-servo-motor-control-with-emc2-part-2-2/
Motor, Gears and Rotary Encoder Wheel