Chagrin:
If you're looking for a simple technique I'd suggest a line-generating laser mounted above your camera and pointed at the floor about a meter ahead of the robot. Any obstacles will show up with a red line on them, and it's relatively straightforward picking out that red line from the image.
Actually, it's better to have the camera facing "downward", and the laser line generator facing forward - that way you can do 3D ranging:
http://www.seattlerobotics.org/encoder/200110/vision.htm
Mount a red filter over the camera lens (if using a red laser - basically, your filter should match as close as possible to the wavelength of the laser) to drop out background light and make the laser stand out more.