Pentax can do this with Full Frame. They can make an adapter for K-mount that recreates the registration distance.
I have both the K-5 and Panasonic GH2. I just bought a M42 Takumar 50/1.4. I put in on the GH2 and not the K-5 because MF on the GH2 is beter. The GH2 is a better camera for MF because of the ability to zoom.
That said, the OVF of the K-5 is more pleasing to look at than the EVF of the GH2. It is reality. But the GH2 EVF can display a live histogram, can display B&W and other effects, and can show DOF effects constantly. I have no doubt that EVF will continue to evolve to a point where it is nearly indistinguishable from an OVF but with the other benefits.
What I'd really like to see in an EVIL camera is a rangefinder mode - essentially a 90% crop of the sensor so that you can see outside of the 90% frame in the EVF. That would be cool.