The imagery in their demo is coming from Google Maps, but if you look at the copyright you can see what companies it is coming from. The USDA shoots, if I remember correctly, the whole country every 3 years, and that data is public domain. The commercial sites are constantly shooting new stuff, but it depends on how often Google wants to pay for it. With their purchase of Skybox though, they will likely start getting imagery updated on an extremely frequent basis.