I downloaded the new Google Earth 4 beta... There is a new layer called "DG Coverage"... Enable it and you see what places are imaged and when (by whom?) and the cloud cover there. Looks like Google uses mostly images that have 10% or less cloud cover. Oldest high resolution images are from 2002.

Several US states and the whole Germany have high resolution images from other sources.

As usual, looks like that the Northern Europe has been neglected... The few places that have high resolution photos are often cloudy and have been taken in winter in shallow light angles...

On a brighter side, National Geographic content has been updated to include the North America.

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Science is a way of trying not to fool yourself. The first principle is that you must not fool yourself, and you are the easiest person to fool.
-- Richard Feynman