Great Thread.
The points about Differential GPS, WAAS are very interesting as this disscusion is moving towards RNP/EPU and RNAV operations. Inorder to fly approaches with RNP < 0.3 the aircraft requires and Augmented GNSS system as raw GNSS cannot guarantee accuracy. in some places RNAV procedures are published with (GNSS) as the stated nav aid required to acheive the accuracy in others it is now being removed. The bottom line is the Aeroplane needs to be able to evaluate the nav accuracy and alert the crew if its not able to meet the RNP.
Getting back to Hand helds or in car systems. I have a unit that knows where the speed cameras are and I use the distance coundown to judge the accuracy of the GPS position. Often as I pass the camera there is still some meters to go or the position is already passed before I get to the camera. The shift from too late to too early can happen in a short distance so that from being too early by 20 meters it can bacome too late by the same distance after just 500 meters travelled. If this same position uncertanty is used to calculate the speed without any smoothing then I wouldn't trust it over the tacho.