Pictures taken from commercial airliners won't be perfect technically, because the plastic windows are so thick they destroy sharpness. Still, the quality is fine for 4- x 6-inch album prints. You can minimize the blurring by getting the lens as close to the window as possible, but avoid touching the surface or fuselage, because the plane's vibration will cause camera shake. Take-offs and landings are the best time to shoot, because the lower altitude makes ground features more recognizable and there's less haze.
In many places you can take relatively inexpensive low-altitude air tours to get some one-of-a-kind scenery shots. Most of the planes have high wings, and often the pilots will let you pop open the window to get clean photos if they know you're a serious shooter. A normal or mild wide-angle lens is good for both commercial and low-level tour planes, but you can use longer lenses to isolate smaller ground features from an open window. In all cases, the exposure recommended by your through-the-lens meter should be very accurate. Depth of field doesn't matter, so use the highest shutter speed you can to offset vibration.
