They speculate about that on the Leica forums quite a bit. My sense is that the M8 is making a leaner, more streamlined Leica company profitable, so I would expect them to release a less-buggy M8x or something in the next year, and probably take several years to develop (and hopefully perfect) the M9.

Whether that goes to a full-frame sensor or not is hot debated. I don't really care, I just wish the digital camera companies would make some better performing and faster prime lenses that worked well at a reasonable price.