The vignetting is a result of the previously mentioned anatomy of an RF. The rear element of most lenses (especially w.a.'s) protrude closer to the sensor in RF's than in SLR's (due to mirror clearance requirements). This forces the light rays fom the lens to strike the peripheral sensor elements (ie. pixels) at a steep angle. This causes a decrease in intensity of the peripheral light striking the sensor and produces the vignetting effect.
The same is true in film cameras. Just look at the photos taken with 12, 15, 16, 21 mm lenses, or in the Xpan 30, and 45 mm lens. Indeed, the panoramic users will use a filter which tries to equate the amount of light striking the "film" by having a central area of darkening "to ensure correct edge-edge exposure." This tries to equate the light striking the film plane in both the center, and edge areas.
So the problem is one of physics, and the sensor is not the problem. Epson uses some software manipulation to try to reduce the effect.
This problem has probably caused the late arrival of digital RF's, and is really a shame since the closer proximity of the rear lens element to the sensor plane has made RF's produce sharper, and higher contrast images than SLR's due to the reduction of internal reflections, and other aberrations.
Hope that this explains the problem. Newer sensors have micro lenses to help focus the incoming light rays on the sensor elements. Perhaps some newer designs will help reduce the problem. Also, edge of sensor increased amplification may help.
Martin