Before we go in to the reason of this strange behaviour lets revisit some basic facts.
1. Sensor of DSLR has an array of micro lenses which focuses the light falling on it to the actual sensor
In the process of focussing light travels from one medium ( material covering microlens) to another ( material of micro lens) and refraction takes place the relationship between angle of incidence and angle of refraction is given by law-of-refraction http://en.wikipedia.org/wiki/Snell's_law
2. If the light ray falls perpendicular on a light sensitive material then the amplitude of generated signal is maximum but as the angle becomes more obtuse / acute ( slanted rays) signal amplitude will reduce.
This is analogous to intensity of sun being more at equator then at poles.
This is how so called "pixel vignetting" occurs.
3. Sensor + Array of micro lens is covered with IR filter / AA filter.
The point 1,2 and 3 were moot in Film era ,
Film has huge dynamic range and angle at which the
Film has higher sensitivity ( and thus huge dynamic range) and the energy loss due to angle at which the light ray falls is not that important.
Now coming back to Range-finder lens they were designed to be of smaller size and for films so typically the rear element was smaller ( compared to SLR lens) and spread light to cover image circle of 35mm film.
Look at the following diagram for understanding this point , I have quickly drawn it in paint so pardon the scale , As you can see angle of incidence at corner for range finder lens is larger scompared to lens with bigger rear element.
This results in 3 things on digital sensor.
1. Pixel vignetting : As the amplitude of light falling at corner is less this results in vignetting despite the image circle of lens being bigger.
2. Color shift / Chromatic aberration :
The simplistic law of refraction are for monochromatic light not all wavelengths are refracted equally .
So as the angle of incidence increases the difference in refraction between various colours becomes more pronounced and visible.
3. Bigger the sensor bigger is the problem so this is visible easily on APS-C and quite dramatic on FF M9
Tail Piece : How some manufacturers claim to reduce this in later models.
My guess : It is not fully established that this effect could be reduced but probably processing power of Camera CPU can be used to do some in-camera processing like you do out-of camera on Photoshop.
Some more examples from NEX5-Nhttp://www.flickr.com/photos/episa/6315302815/