(If you want to see this picture better, go to Mark's blog and click on the picture there.) What it appears is that the PCA is especially strong in the northeast (especially the corridor between NYC and Philly) and in the southeast. We have a strong (and growing) concentration in Chicago and Los Angeles and churches in the major metro areas of Texas.
Now there are a number of things you can conclude from this map. On the one hand, those who see the PCA as a "southern Presbyterian church" have some justification for this. Statistically, there are simply more PCA churches in the South than anywhere else. Another thing that you could say is that the PCA is showing a greater awareness for planting churches in metro areas throughout the country--Los Angeles, Houston, Dallas, Indy, Chicago, Denver, Seattle, NYC, Orlando, etc., all have healthy representation of churches.
But the thing that strikes me as a historian who does about Presbyterianism is this--our strength lies exactly where Presbyterianism has historically been strongest. If you know anything about Presbyterianism in America, you know that the centers of Presbyterianism have been places like Philadelphia, Chicago, NYC, Charlotte, St. Louis, Atlanta. In fact, if you think just where the old PC(USA) had seminaries, we have a strong presentation of churches in those places. That certainly has to mean something.
I think it means that, while we certainly have a great deal of work to do to reach the west coast, the fact that Presbyterian churches are flourishing in the parts of the country where you'd expect them to do so should give us great hope as well. People who grew up believing that Presbyterians stood for solid doctrine, careful government, reverent and biblical worship continue to flock to that same kind of Presbyterianism, even in a younger version (such as the PCA).
But another thing here are the oddities. For example, historically Virginia has been strong Presbyterian country, but there are relatively few PCA churches in the state, save along the I-95 cooridor. We simply have to do a better job planting churches in northern Virginia and in the Valley; as we continue to do RUF in the state at the major universities, church planting should proceed apace. In addition, there is a huge blank space in the state of Missouri; this has been good ground for Presbyterianism in the past, but our efforts have been restricted mainly to St. Louis (even Kansas City has relatively few PCA churches). Other parts of the Rust Belt are similarily sparse--Ohio, Indiana, Michigan all are fairly "light" when it comes to the PCA.
Still, this was a fascinating map that should encourage us as well as motivate us to continue to seek God's Kingdom in biblical, Presbyterian churches throughout our entire country.