I'm not 100% sure that is true Peetster.
While theoretically it is easier to contact those in the city, the theoretical method of polling is to get a sample size (say 1000) people by randomly choosing 1000 people in an entire state. They just randomly choose and so, theoretically, it should be representative of the state's distribution as well (as in if 5% of the people live in rural areas, then 5% of the 1000 should be represented).
Of course this is all theoretical but that is why there is a margin of error. That is why anything <9% is considered weak because a 4% margin of error means in a state where Kerry leads by 8%, he could actually be tied with Bush.
And much of poll work is now done via phone for instance and so it's not particularly hard to just call someone living in the heartland.
Besides, how would anyone ever contact someone in Kansas
