Unfortunately, the mass effect of white roofs is not as effective as with an individual building because the reflected sunlight still warms the urban area, creating a larger, warmer urban heat island.
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Well that certainly explains the balmy temperatures during antarctic summers.

Not many roofs in Antarctica. :-)

No, the idea is that while a white roof will keep your building cooler, the total benefit for a city is less than the sum of the individual buildings. Anytime you can see a roof, either from street level or from another building, the reflected light carries some energy. A white roof reflects more light and thus more energy to surrounding surfaces (and clouds).

In addition, there is some modeling the indicates there is little benefit for global warming; other work indicates the opposite. I would guess the reduction in cloudiness is related to the reduction in precipitation noted earlier in the thread.

http://www.eurekalert.org/pub_releases/2011-10/su-ui101811.p...
One "geoengineering" proposal for reducing the impact of urban heat islands is to paint roofs worldwide a reflective white. Jacobson's computer modeling concluded that white roofs did indeed cool urban surfaces. However, they caused a net global warming, largely because they reduced cloudiness slightly by increasing the stability of the air, thereby reducing the vertical transport of moisture and energy to clouds. In Jacobson's modeling, the reduction in cloudiness allowed more sunlight to reach the surface.

The increased sunlight reflected back into the atmosphere by white roofs in turn increased absorption of light by dark pollutants such as black carbon, which further increased heating of the atmosphere.

Jacobson's study did not examine one potential benefit of white roofs – a reduced demand for electricity to run air conditioning in hot weather. But a recent study done at the National Center for Atmospheric Research showed that the decrease in air conditioning use, which occurs mostly in the summer, might be more than offset by increases in heating during winter months.

DB2