The latest Steam client beta contains a mockup image of what appears to be the new look of the Steam Controller.
ValveTime has dug up the image of the new design from the latest Steam beta client.
Other than a few ergonomic enhancements, the biggest change is the addition of the directional indicator on the left touch pad. The analog stick that was added in July is still there, which could suggest that the final design is near completion.
At any rate, we expect Valve to show off the latest design - and give everybody more insight into the Steam machines - at GDC next year.