Luxury-spending data can tell us a lot about the state of the nation—and our own neighborhoods.

Take Detroit—not the city where one might expect to see the strongest recovery. Yet when American Express Co. looked at luxury spending in top and midsize cities around the country, Detroit led the list, with growth of 18% in the first quarter of 2010 from the year-earlier quarter. Lo and behold, Ford stock is up, too, suggesting that Detroit's local investors are feeling more optimistic than they were when auto executives were...