the recent smart phones seem to be quiet good for gps tracking. Accuracy sometimes not perfect, but even my Locosys GT-31 accuracy fails sometimes by 10-20m, especially in the mountains. Yet with the new Bing-Ovelay in josm it is much easier to find this kind of problem and create extremely accurate mapping.

Yes, today you have to search for the areas where the map needs work. Yet I found a lot detail work still needs to be done, many small villages still badly mapped, or the mountain tracks in the Alps sometimes are not very accurate, sometimes straight wrong.

BTW, the Nokia N900 mappero software shows you the accuracy of the gps as a circle on the online or cached map.