As Jeremy points out, the micro four-thirds cameras already have an adaptor to allow use of M mount glass and a third party adaptor to allow similar functionality for the Sony NEX line has been announced as well.
would ever have the resources to develop a rangefinder with a digital sensor is a moot point. Leica did have the R&D resources but I doubt there'll ever again be a mass market for rangefinders which begs the question of why anyone else would bother.
Like Bjorn I'd love to own a Leica M but in my more rational moments I can't help feeling that Leica are in a tough spot. Either continue as they are with marginal improvements to the M9 or rethink the mount. If the latter then at the very least including two way communication between the lenses and the body together with a camera controllable iris would seem possible without adversely impacting the size of the lenses but I freely admit that's a guess on my part. It might even be possible to do so and retain full mechanical compatibility between old and new lenses and bodies. Whether including AF in M mount lenses is doable as well, again bearing in mind any size implications, is above my pay grade but Leica have demonstrated they have the know how with their S system.
I can already hear mutterings about the need for all this with a rangefinder camera. After all, the argument would go, being a rangefinder is the whole point of a Leica M. Well, is it? Not having played with one I'll admit some puzzlement about why rangefinder focussing is any better than split prism focussing. Not that a split prism is an option on a Leica M for obvious reasons but with the new generation of high resolution electronic viewfinders coming along I'd argue that for the future Leica could reasonably ditch the rangefinder for an EVF system which would offer all the visual cues needed for manual focussing. You wouldn't be able to see outside the frame through the viewfinder and that would be an issue for some but at least an EVF can be positioned so that the left eye can see past the camera while the right eye looks through the viewfinder. Or is that a step too far for the Leica aficionado?
To work well an EVF system would need lenses with a camera controllable iris, as I mentioned earlier, and if AF could be included as well then that just adds to the ways the camera can be used. But neither facility is, so far as I can guess, an absolute requirement so the older glass could still be used. The kicker is that by following this evolutionary path the M line would become just another EVIL camera, albeit one right at the quality end of the market.
So, dragging myself back to the thread topic, rather worryingly it's possible to argue that no third party manufacturer will find it worth their while to develop a digital rangefinder to work with existing Leica M glass and, for the same reasons, Leica will hit a cul de sac as evolving the M series into a "me too" EVIL camera may bring them into direct competition with micro four-thirds and similar lines. But just continuing with rangefinder cameras and marginal improvements in sensor performance will look increasingly anachronistic to the next generation of potential owners that Leica will need
if the M system is to survive.
Perhaps they've already seen the writing on that particular wall and will move more and more of their R&D focus to the S system.