I asked for coverage maps and I got them, at least in part. This page from Sprint shows the locations of their towers across the country. You can drill down right into your neighborhood.

The problem is that the maps don't show signal strength in various areas, just tower locations. You might still get surprised by dead zones. The friend who sent me the map noted that even though there's a tower close to his office, he still doesn't get great reception. I'd also like to be able to overlay one service provider's map on top of another's. Of course, to do that, it would have to be done by a third party.