Selective geographic marketing

Charlie Stross floats the idea (via Warren Ellis) of only marketing to "blue" states in the USA. Iteresting idea, but I'd want some good statistics before I ignored potentially half of my readers. Not sure the same thing would work in the UK, get a book in a display in WHSmiths or Waterstones and you're sorted. Plus everyone is used to writers only touring in cities and the nearest city is never that far away (unless you live in an outlying island (Isle of Wight? ;-) )