I met an aerial photographer last week and here is some information I can share from the conversation. At a height of 6 to 7 kilometers, you could practically get pictures with a resolution of 1m. Pictures taken from aircraft for surveys usually are taken from multiple cameras and then composed together to make an image. Image sizes easily reach a few tera bytes. And then for some special purposes, people fly as low as 1 km. Imagine detecting which song is played on your friends iPod from a kilometer above. Things could get a little on the embarrassing side too, but that would not be the first time technology tripped some one (Ever left your webcam on or typed on the wrong chat window ?).
Another random fact:
Once in a while, satellites capture aircraft in flight. You can watch some of them through the .kml file available on this page. Ofcourse, you need Google Earth!