Mayor Anthony Williams would like to add 100,000 new residents to Washington, DC, whose current poplulation stands at about 572,000. This is certainly physically possible, as the Washington's population peaked during the 1950's at well over 800,000. Since that time, the population of D.C. proper has shrunk by a third, but the speed at which the decline is taking place has slowed significantly. The signs of gentrification and revitalization of blighted areas is obvious in many parts of the city once abandoned as slums, including Logan Circle, Shaw, and Columbia Heights.
I think it's possible. Among the dense, older cities in the U.S. that actually gained population between 1990-2000 were Boston, New York, and Chicago.