I wonder if in removing the mirror box they encountered the same issues as leica with the M8 or perhaps the sensor is small enough to avoid them.
m4/3 lens designs should be able to avoid the problem that some short focal length rangefinder lenses have with electronic sensors. The problems for those Leica wide-angle lenses is that they use near-symmetric lens designs, which are optically nice and more compact than SLR lens designs of equal focal length, but have a low exit pupil height, meaning that light reaches the corners of the 35mm frame at a substantially off-perpendicular angle: maybe even more than Kodak's off-set microlens approach can handle.
But most digital cameras are already mirrorless (all the small sensor compacts plus the Sony R1 and Sigma DP-1 and DP-2) and these often use lens designs that are more compact due to allowing the rear lens elements to sit close to the focal plane while still having a high enough exit pupil to work well with the sensors of those cameras. These lens need to be somewhat retro-focal at wide angles, so are not as compact as traditional rangefinder wide angle lenses, but there is still more design flexibility than with an SLR lens. The lenses for the Sigma DP-1 and DP-2, a 16.6mm f/4 wide angle and 24.2mm f/2.8 "wide normal" respectively for Sigma's 25mm diagonal sensor, might give a hint as to what is possible with small primes for mirror-less cameras using DSLR sized sensors. But I am far more interested in smallish zooms, like a 2x wide and a 3x standard.
Anyway, the bottom line is that eliminating the mirror only adds design options that SLR's do not have, it does not rule out any design choices, including simply using existing SLR lens designs in some cases (like telephotos): m4/3 lens designers simply have more options and trade-offs to choose between. I can imagine some entry-level m4/3 lenses being designed to be as small, light and inexpensive as possible, at the cost of relying on some software correction for light fall-off towards the edges of the frame --- after all, that is good enough for Hasselblad with its 28mm! Higher level m4/3 lenses could instead favor stricter optical performance standards at the cost of being less compact.
