It's pretty clear the sensor is unchanged. It's not an all-new, improved sensor. Much as I'd love a new low-noise sensor, with clean files up to ISO 25.6k at 6mp, it's the same pretty darned good sensor.
The image processing engine is new, called EDiART. I've seen it mentioned EDiART will speed RAW processing. I haven't heard whether there are other changes/improvements to image quality due to EDiART.
It looks as though the R-D1X is already available in Japan. Has anybody anywhere posted results from a production camera?
As to R-D1/R-D1S firmware upgrades, I've seen third-party sites mention that SDHC support can be added to the R-D1/-D1S through a firmware update, but I haven't seen any actual firmware updates anywhere. Epson USA and Epson Japan have no firmware/software downloads newer than 2007. I haven't heard mention of any other potential updates.