I think that what you're seeing is the shrinking of the 'serious' camera. With new technology that will allow more sensor surface to be dedicated to light capture, less to wiring, signal/noise ratios will improve.
There is news from Sony that could be very promising in this direction, even though initially it is in the form of a 1MP camera phone sensor: http://www.eetimes.com/sys/news/OEG20040305S0031
What excites me is that this sensor is a frame transfer CCD instead of the interline transfer CCD common in small sensors (see the glossary at http://www.roperscientific.com/library_glossary.html
for details of the difference.)
Frame transfer style reduces the amount of non-light sensitive area needed on the sensor surface, thus increasing sensitivity and perhaps well electron capacity. This should lead to higher usable exposure index (miscalled "ISO speed") and perhaps higher dynamic range at a given pixel spacing.
Overall, I see 2/3" format becoming more and more satisfactory for the main image quality needs of the great majority of even moderately serious photographers, so that the most common reason for larger sensor sizes will be for the various consequences of access to lenses with larger maximum effective aperture diameters. In particular
a) lower minimum depth of field,
higher speed through gathering more total light from a given subject in a given exposure time
c) from that same greater light gathering ability, more light for an optical TTL viewfinder, to give a bigger and/or brighter image: electronic sensors might be getting more sensitive, but our eyes aren't! The only 2/3" format cameras with optical TTL viewfinders so far, the two Olympus E-xx models, seem a bit limited in viewfinder image size and brightness, though their use of a beam-splitter in place of a reflex mirror hurts a bit. Maybe good enough EVF's will someday make this item irrelevant.