Check out this map. It shows areas that lack 3G (or better) access. Data comes from the FCC.
The map’s description says:
This map made with data current as of January 2012. This map shows the areas identified as potentially eligible for Mobility Fund Phase 1 support. These areas are US Census blocks that lack 3G or better mobile coverage at the centroid of the block according to January 2012 American Roamer data and contain road miles in any of nine road categories. Counties that contain any of these blocks are shaded light gray, and as you zoom in and mouse over these counties you will see more information on the potentially eligible blocks, including population, road miles (S1100, S1200, and S1400 categories only), and the name and number of the CMA in which the blocks are located. Further zooming in allows you to see the US Census tracts that contain these blocks.