Richard,

  In older processes, the bulk substrate concentration is different for
the n-channel and p-channel transistors. Usually one device is built
into well, while the other exists in the substrate. The well doping
needs to "counter-dope" the substrate doping and is typically ~10x
higher in concentration. As a result, the depletion regions change at
different rates with applied bias for n-channel and p-channel transistors.
The difference in the rate of change of the Drain-Bulk depletion region
is a significant contributor to the difference in the channel length modulation.

                                                                  Best Regards,

                                                                     Sheldon

Berti,

  It should be a reasonable first-order approximation.

                                                    Best Regards,

                                                        Sheldon