I saw this at Flowing Data -- this is apparently a chart prepared by some sustainability group at MIT to map solar potential of different sites in Cambridge, MA
Look at all the sites marked "excellent". I have news for the brilliant folks at MIT. Even the best, flattest roof facing south in Cambridge, MA still rates a "sucks" for solar potential. (source)
Even with massive state and Federal subsidies, those of us who live in the bright red areas find that roof-top solar PV is still an - at best - marginal investment with very long payback times. We all hope to change this in the future, but there is no way a city like Cambridge with approximately half the solar insolation we get in AZ is going to have "excellent" roof top solar PV sites.