They want to use so-called statistical sampling in the census and use estimates to produce “official” census numbers in 2010.
You can imagine the political pressures to estimate that there are more people in East L.A., the South Side of Chicago, Harlem, Bedford-Stuvesant, etc. Add a few “estimated” residents here and there, and, suddenly, California gets one or two more congressional seats than they should have, Illinois gets more, New York gets more too. Blue Democrat states will get estimated residents which will increase their congressional representation and federal funding.
If there was ever a government function that should be totally non-partisan and bureaucratic, the Census Bureau is it.