To understand the size of this issue, I tried a visual example with two images. One was already published in the blog. It was a series of blind shots (because the stars were almost not visible at plain sight), 12 lights and 3 darks of 20 seconds exposure with the kit lense of the Nikon D3100 at 18mm f3.5. The light pollution was, of course, present as this photographs were taken on a "rural/sub-urban" area around 20Km from the nearest city.
I wanted to run a test and try to take the same frame from a different location, a truly rural location with very little light pollution (this time it was possible to see the milky way even at plain sight so it was easy). This location is around 40 Km from the previous one. The shots were the same with the same lense: 12 lights, 3 darks 20 seconds exposure 18mm at f3.5.
As you can see, the light pollution is a true enemy for the astronomy and, of course, for astrophotography. Next time when you are outside, just look over your heads. If you're in a city you'll be lucky if you can see a bunch of stars...