This NYTimes article has the phenomenon exactly right, as to the potential to the user (actual and assumed power over the map), the opportunities for the geoweb vendor (billions in advertising dollars) and the challenge to the cartographer and GISer (whence the role of the expert).
[M]illions of people are trying their hand at cartography, drawing on digital maps and annotating them with text, images, sound and videos.
In the process, they are reshaping the world of mapmaking and collectively creating a new kind of atlas that is likely to be both richer and messier than any other.
The article has some nice links to geolocations on Google Earth (e.g., biodiesel stations in New England). You should visit the URLs and save them before they disappear behind the firewall.
More on this, as soon as I finish reading Andrew Keen.