If it is required to put the professional looking pictures and videos capturing ability to mobile phones and tablets, Sony's efforts will be really fruitful. This step by Sony will surely be highly result oriented as Sony team have a very vast experience for these sensors due to their Professional Video Division.
Sony image sensors are of very high quality and have unique result. However, they are not freely available for design and prototype work. They should also make effort to make literature freely available and supply samples for designs. Will they also go for advance ASIC to support these CMOS image sensors?
A smaller sensor does not necessarily mean better quality, for an image sensor, the most important parameters is Fill Factor and efficiency. A smaller sized pixel would mean reduction in these factors.
The back-side illumination is the approach to cope with fill factor. However, I agree that smaller sensors is not generally the goal.
Bigger pixels, more pixels, which usually means more die area and cost.
One way in which smaller die might help is if a developer goes for multi-sensor approach similar to Pelican Imaging.
I don't know how much more affordable they can get! To a first order, the higher end cameras essentially have larger area sensors. It's really pretty amazing how far things have come in the last 10 or so years. The camera modules in phones are a few dollars. Not convinced? You can get a replacement module for the iphone 4s for around $12. (anyone guess country of origin?_