Andrea Nill of the Center for American Progress offers "Why Counting Undocumented Immigrants In The 2010 Census Counts For A Lot" (wonkroom.thinkprogress.org/2009/08/10/immigration-census-bureau), a through-the-looking-glass post listing all the wonderful ways that states and localities can make out like bandits from counting illegal aliens in the Census and using their population in apportioning Congressional representation:

...the census serves many other purposes, namely the allocation of scarce federal resources for states and localities. Census data is used to distribute federal funding and Community Development Block Grants that benefit all residents. In a recently released report, the Drum Major Institute (DMI) shows that not counting undocumented immigrants would lead to inaccurate demographic information and result in costly mistakes in infrastructure, education, and healthcare planning...