Mark Norton over at L-Camera-Forum (registration required) attempts to answer this question by inserting a full-frame sensor mockup (composed of a hand cut aluminum sensor frame with a 36 mm x 24 mm opening and a ground glass insert) and then checking for any coverage issues by mounting various M lenses and looking through the ground glass.
The current internal arrangement of the Leica M8 includes a plastic throat that causes some lenses to vignette (more so with telephoto than wide-angle lenses). Mark concludes that in order for the M9 to support a full-frame sensor, the body will have to be slightly larger or the battery will have to be made smaller in order to accommodate a larger lens throat.
The consequences of a larger body means that a whole new set of top/bottom plates will have to be designed along with accessory grips and third party products will have to be adapted as well. A smaller battery may mean less run times unless the new Maestro chip consumes far less power than current processing chips.
Kudos to Mark for his amazing work.
Read it all here.