Until recently I was on the road covering all of the lower 48 states. I'd deliberately try to find rural locations to park for the the night, and either just howl for response, or if circumstances allowed, try to call something in (no rifle, so I wasn't hunting).
There were very few areas where there weren't coyotes, though responses were more rare east of the Appalachians, and most common (in general) west of the Mississippi.
I have called in coyotes (not just gotten a response) in industrial sections of a number of cities, L.A., SLC, Dallas, Albuquerque, El Paso, etc. These were often areas where there basically wasn't any bare ground, all concrete and asphalt.
There are A LOT of coyotes around, many more than most people think there are. Coyotes are at or pretty close to the top of the food chain in most areas, and it seems to be a law of nature that their population will expand to meet the available food supply, whether that's rabbits, mice, etc., or McDonalds dumpsters and alley cats.
I suspect though, that the underlying question behind Verg's post is "where are coyotes easiest to hunt", and that's a whole other ball of wax.
Other than Texas, where coyotes are stupid (Tank says so /ubbthreads/images/graemlins/smile.gif), I think how difficult it is to call in coyotes has much more to do with pressure than population.
Here in the valley (Phoenix area), there are lots of coyotes just about everywhere, but they can be very difficult to call in. I've been in areas where there is so much scat and sign you almost had to step around it, and not seen a coyote all day.
Hunting is open on just about all land here, even private land unless it's posted (most isn't), and between the predator hunters and the quail, javelina, deer, etc., hunters, the coyotes here see a lot of pressure.
I'm not an expert, and rarely hunt really hard, but I probably average only about one coyote for every two days of hunting here in the valley.
Contrast that with coyote hunting north of the Canadian river in Quay/Harding/San Miguel counties in NM, where all the land is private and can't be hunted without permission, and where nobody normally hunts specifically for coyotes (though shooting them when you see them from the road is expected).
In an informal "winner takes all" contest there, my partner and I shot 20+ coyotes in a day, and didn't win.
Granted that was a summertime hunt so a lot of stupid pups were taken, but even the adults weren't particularly cautious, and while there is certainly a higher population density of coyotes there, it's not 40 times what it is here around Phoenix.
The difference is that there are only about 1500 people in a 8000 square mile area, and none of them normally "hunts" coyotes.
If you want to kill a lot of coyotes, go where there's very little hunting (for anything). You'll do much better looking for low pressure areas than high coyote population areas.
Of course an area with both low pressure and high coyote population density is best, but we can't ALL hunt in Texas. /ubbthreads/images/graemlins/smiliesmack.gif /ubbthreads/images/graemlins/grin.gif