There are really only two companies with accurate data. Navtech and Teleatlas. They both employ a lot of people driving out on the roads. The Navtech system sends out drivers in teams of 2. One drives and the other operates the laptop, designating roads that are closed, new roads, designating one way streets, private streets, etc. Through this technique I think they're able to cover nearly all the drivable streets in the major metropolitan areas. The non metros are left to government data and satellite maps and such.

The way the data is listed, is interesting, they have each interesting segment of road given in a vector type format that correlates closely with geographical coordinates. I'm sure that if you "stole" the data they can easily identify the origin of the map data by comparing segment/vectors. Since the segment vectors roughly correlate to actual physical drivers moving around the roads, no segments from two different companies will ever precisely match, is my guess. Also, NavTech and TeleAtlas does not cover the exact same geographical areas. They're expanding into other countries at different paces, so some data will be "blank" from one company and not "blank" in the other.

Calvin