All, I work for an aerial photography company which has supplied imagery to both the US government (thus into Virtual Earth) and Keyhole (through APUSA).
All of the high quality imagery is from airplanes. You are right in saying there is a big difference between different areas -- it takes a lot of expense to get high quality photos of an area. You can purchase imagery from other sources for higher quality and more up to date details.
Quality of imagery is not just related to the level of detail. You also should know when the photo was taken. Some areas are changing rapidly and the imagery can be old. I have noticed gross changes in GoogleEarth, where I knew the photos are more that 8 years old by what was shown, but there is no way to tell it on the interface. The date of the photo should be included on everyone of these websites because it really impacts the quality of the photo.
Feel free to write me directly if you have questions or want some help in this field. George -- George@ikcurtis.com