Despite the fact that 35mm sensor has a superior performance compared to the smaller counterpart, the market of the latter one in growing. With the recent introduction of Canon D30, Leica just announced their joining to the 4/3 system.
It does increase the pressure to Sony to have Zeiss support for their new system, perhaps they are working together behind the curtain right at the moment. The big question is what size will be for the new sensor ? Will it be similar to R1, 30D, 4/3 or will it be like 5D ?
At the moment the smaller sensor market is a lot bigger than the full frame one, if Sony join this part of the market she will be competing with many companies. I guess Sony would not mind as she is fully capable of doing so.
Sony has not made a 35mm sensor, for her to introduce this system, it would be a big jump for her.
Perhaps she could introduce two systems, both the full frame and the smaller one and this is indeed what Canon is doing now.
As a fan of both Sony R1 and the Leica M system, I can certainly realize the importance of being mirrorless. The importance of the Sony evolution history is that they like to stay mirrorless. From 717,828, to R1, the sensor size increases but none of them have a mirror.The mirror is certainly more important in the full frame than the smaller frame. With a small frame, you would find it less useful than the bigger one. R1 is perfect in wide angles, but it is a bit slow in normal and telephotos. It would be sensible to deduce that Sony could introduce the mirrorless, interchangable lens system with the smaller frame to compete with the others. This system will be superior to others that it is being more stable to mechanical vibrations and has unbeatable performance in wide angles. With interchangable lenses you will also have good performance in telephotos.
For customers who would like to see the mirror, Sony will introduce an SLR with the traditional 35mm sensor size. In this system, Sony will continue to excel in wide angles. From the history we understand that Zeiss provide superior performance in wide angles, no matter the mirror is presented or not.
The reason why Sony should introduce these two systems is because in the digital market, you have to produce waves of products to flood the market, this is how Canon is now playing the game.