[ericmj]

Yes. But the "constant factor" will be very large, even larger than for traditional ray tracing, for some global illumination algorithms, even when you chose to only render a small part of the scene. For example photon mapping requires a pre-pass where you trace photons from the light source onto the scene, that means that reducing the pixel count doesn't decrease the time for the pre-pass. http://en.wikipedia.org/wiki/Photon_mapping