Google maps explicitly states that if you embed Google Maps on your site and expect heavy traffic you have to notify them.
This does not stop Manifold from using it in their application and boasting about it. So what is the acceptable use policy with regard to Applications/Desktop clients using Google Maps data. It must be somewhere. If you leach too much Google will die on you and claim that you are a virus, which they are right about since bandwidth is being chewed up by freeloaders eager to see theirs and everyone elses house.
But, if an Open Application such as Worldwind transparently uses this data with acknowledgement and brings it to more people who probably cannot use Google Earth due to low bandwidth connections or want to operate in remote areas in offline mode, it should be encouraged since it brings the benefit of this great dataset to more important uses such as Emergency Response rather than just the Desktop Globetrotter. The community can also add bandwidth they have bought to the Dark Fibre Google owns to further ease the use of the imagery.