Have been experimenting with simple mirrored stereo vision, as described on the Mirage Robotics site. It's fairly trivial to adjust the stereo sensor models to handle this type of setup. The good news is that this effect certainly works, and would simplify the camera calibration procedure. However it does look as if you need quite a large mirror in order to be able to get a good stereo effect, which limits the practicality of this design. This would be ok if you needed stereo vision on a large robot, but I think for most smaller robots it wouldn't be suitable.

So for the present it still looks like conventional twin camera stereo is the preferable option.