36 megapixels is exactly the same pixel pitch (4.73 um) as the 16 megapixel APS-C D7000, which is a little bit smaller than the phase one IQ180 back at 5.17 um, and a bit larger than Canon 7D at 4.16 um. 36 megapixels on a fullframe sensor is not an extreme amount, one would rather say that the current fullframe sensors have compared to other formats extremely high pixel pitch.
One should also take into account that the larger the image circle the more difficult it is to make a lens resolve small pixels, so you can probably have a little bit smaller pixel pitch on 135 fullframe than on medium format.
Putting a full-frame lens on a D7000 and you can see how the center portion of the rumoured D800 sensor will perform. It works well, so I'm certainly not too worried about lens / focus performance. Before edit I said you could see autofocus performance too, but that is the old mistake of mixing up pixel size with resolution. With 36 megapixels fullframe you would need longer focal lengths for the same FoV and get shorter DoF and need more precise autofocus. Will it be good enough? Don't know, not too worried though. When I myself use autofocus it is for hand-held action, and then resolution is limited by that I'm holding the camera in my hands anyway... on a tripod I use live view-assisted manual focus almost all the time.
Say if you need very good corner performance, zooms with wide angle will probably not perform that great, neither will extremely wide lenses (wider than 24mm). But primes at f/8 will most likely give you very good performance corner-to-corner, and tele lenses will probably also work well.
And that field curvature "problem", is it really a problem for anything else than shooting flat test charts?