It was rumored a few months back that Microsoft had a system configuration that put the old Xenon motherboard from returned 360s with the Falcon chip set featuring the 65nm CPU and 90nm GPU.  This new configuration would not have a HDMI port but would use the new improved CPU and GPU chip sets.  This model will run cooler and has a higher likelihood of never needing to get returned for service again.

It seems that Microsoft is shipping back repaired refurbished system made from spare parts from broken 360s.  Since the issue was not the motherboard but actually the GPU that was causing these systems to get the three red lights it seems practical to reuse the parts.  I think that since Microsoft waited this long to introduce the Opus configuration it's safe to say that the new GPU and smaller CPU have done their job and decreased the amount of returns.

Source:
xbox360fanboy.com - opus-model-360s-begining-to-flow