Here is some quick figures from City-Data.Com.
While Detroit certainly isn't the only city shrinking it is shrinking at a rate much higher than other major cities.
Philadelphia, Pennsylvania
Population (year 2000): 1,517,550. Estimated population in July 2006: 1,448,394 (-4.6% change)
Chicago, Illinois
Population (year 2000): 2,896,016. Estimated population in July 2006: 2,833,321 (-2.2% change)
St. Louis, Missouri
Population (year 2000): 348,189. Estimated population in July 2006: 347,181 (-0.3% change)
Detroit, Michigan
Population (year 2000): 951,270. Estimated population in July 2006: 871,121 (-8.4% change)
Los Angeles, California
Population (year 2000): 3,694,820. Estimated population in July 2006: 3,849,378 (+4.2% change)
Atlanta, Georgia
Population (year 2000): 416,474. Estimated population in July 2006: 486,411 (+16.8% change)
Portland, Oregon
Population (year 2000): 529,121. Estimated population in July 2006: 537,081 (+1.5% change)
Boston, Massachusetts
Population (year 2000): 589,141. Estimated population in July 2006: 590,763 (+0.3% change)
Phoenix, Arizona
Population (year 2000): 1,321,045. Estimated population in July 2006: 1,512,986 (+14.5% change)