My thinking is that there had to be a sensor change. For one thing. S3 is supposedly doing images at ISO400 which look like the S2's ISO200 images. That being the case and the fact that they can now squeeze ISO800 and 6MP, I gotta believe the S3 has a new censor.
But like you said, what do I know. But if you ask me, by changing the model name to S3, its easily possible that they could have changed the sensor.