If you take a quick look at the basic elements of replacing a strip of film with a electronic sensor, then it will become clear that this is a very different issue on a rangefinder compared to a SLR
This issue is well illustrated both by physics and also the presence of 3 camera models which all accepts Leica M lenses.
Read more after the break.
There is a big difference between film and a sensor, in this particular case the most important difference is that film is FLAT, unlike digital sensors which have pits or little holes.
To get a idea how this matters, lets abuse your best friends pool-table (this is better than using your own). first draw a grid of about 3000x2000 little squares. then hold a laser pointer about 4-5 feet above the surface of the pool-table, shine it at the squares at the edge of the table and then at the center.. you can easily illuminate each square because the table is nice and flat..
Now let us turn the pool-table into a digital sensor.. grab a few thousand shot glasses from the bartender, and align them on the table so there is one shot glass in each square. Now repeat the laser pointer experiment, again center the laser pointer about 4-5 feet over the table and start pointing at each square (or rather each shot glass) the point being to illuminate the BOTTOM of each glass. you will quickly discover that the its easy in the center of the table, but towards the edges this quickly become more and more difficult.
So we now know that it does not take a rocket scientist to figure out that the shot glasses by the corner pockets are pretty much impossible to illuminate to the bottom of the shot glass.
BUT the solution is easy, instead of holding the laser pointer at 4-5 feet, you could hold it at something like 10-12 feet and now it is easy to see the bottom of all the glasses.
This experiment illustrates the biggest optical difference between SLR and Rangefinder lenses. SLR lenses have a large distance between the rear lens and the sensor, this is because SLR cameras was made to make room for a moving mirror between the lens and the film. Leica M rangefinders do not have a moving mirror, and this permit better optical design of shorter lenses, the result is that the end lens of a Leica lens can easily get as close to the sensor as 10mm (compare to about 40mm for a SLR).
Finally the last problem for Leica. over 50 years of perfect lenses. All of those made for film and therefore almost impossible to use with a sensor full of pits.
Leica overcame this issue by installing micro-lenses on each sensor pit. and because of this design it is possible to use Leica M lenses with the M8 digital rangefinder. But keep in mind that the M8 is not a full frame camera but have a 1.3 crop factor. going from 1.3x to full frame with today's sensor technology, is not practical in a digital range finder. However there is no doubt that future technology will solve this and a we will have a future Mx camera with a full-frame sensor.
Even with the crop factor Leica still had some vignetting left in the camera after installing the micro-lens solution. Leica solves this by applying a software based lens treatment in camera, this is possible because Leica uses a 6 bit code to tell the camera what lens is mounted.
New Leica lenses have the code already from the factory, however older lenses can either be send back to leica for coding, or in most cases this coding can be applied with a sharpie by the user. I have a template available for this here.
Currently there are 3 digital cameras on the market which accept a Leica M lens.
Leica M8 & M8.2 this camera have a 1.3 crop factor and over come the corner pocket problem by adding a micro-lens to each pixel, this is still not a perfect solution because even with the little round lenses at the corners, it is hard to get all the light to the bottom, as a result Leica does some in-camera processing to successfully reduce vignetting.
Epson RD1 This was the first digital rangefinder camera, Epson used a smaller sensor than the M8, the RD1 sensor have a 1.5 crop factor and even at this factor vignetting can be a issue.
Panasonic G1 and future Micro four-thirds cameras. Finally the newest camera, this camera have a even smaller chip with a crop factor of 2x.with the smaller sensor size vignetting is not as big a issue, but interestingly with short lenses less than 28mm there is a noticeable drop in sharpness at the edges.
The Panasonic G1 and M43 in general, is interesting because this is a mirror less camera, and have a smaller distance from lens to sensor of about 20mm.
The M43 lenses have a interesting design, made to direct the light more directly into the sensor. Unfortunately this is a solution Leica can not take advantage of, because Leica is committed to supporting all M lenses since 1954, therefore the solution must be strictly in the camera and not by redesigning the lenses.