It looks like the urban areas are photos and the rural areas are landsat. You can really tell inthe resolution. Get outside of the city and the photos are really pixilated.
When I checked Sacramento there are buildings pointing different ways, this is the sign of a mosiaced (sp?) aerial photo. I checked SF and the buildings all point in the same direction that one is a sat image, probably spot or something with high res.
I made a global assumption when looking at Sacramento, it looks like there are multiple sources for thier data. A nice assemblage for a free service.