Google owns their own map data for the entire US and Canada, and is quickly getting other areas too. Their streetview cars collected enough data to let them break ties with the other providers.

Easiest way to tell is to go to maps.google.com and look at a location with the map only view. If the copyrights in the bottom right are only for Google, thats an area using all in house data. Only when I get close to Mexico do I see something other then Google pop up down there.