IMO the ideal situation would have been an open standard for a lens mount to suit an APS-C sensor due to it being the best compromise of size vs image quality (again IMO).

Sony have shown that it is possible to create a very compact camera around this size sensor with the NEX3/NEX5.

Also most existing lenses would fit with an adaptor (possibly even retaining AF/aperture adjustment) due to the reduced flange to sensor distance, as they currently do for m4/3.

While there is a lot of momentum behind m4/3, I personally am left feeling that it is a 'test run' to refine the mirrorless system for a future larger sensor. Look at how little love there is for the original 4/3 standard. However if Canon or Nikon back it, it will certainly become the default standard but as was previously stated, the 'big two' will want to try to lock users in to their own lenses only so them backing m4/3 looks unlikely.