It is indeed hard to know what is done, by the manufatcturer, on the sensor. On the other hand, if one beliefs the results of Dx0, and look at the Nikon D3s, D3x and d7000 (in screen mode, the results per pixels), the dynamic range increases as the pixel size decreases.
The method with multiple exposures per pixel, if the light intensity is high, seems less likely on the type of sensor used in a DSLR. All of them , as far as I know, need a shutter, at least to terminate the exposure.