The Census also dramatically undercounts privacy oriented libertarian residents (and their info). Most anti-gov and privacy folks don't send any info in or not enough for the Census Bureau's software to check for double counting.
If folks want to be counted (to help with representation), you can just supply three pieces of info per person (age, gender, relationship to owner, for example), and the software will include the April 1, 2010 status of that house/apt.
It's important to mail something in because liberty-leaning State House districts will find themselves with fewer State Reps--over the next ten years.
Census also does regular statistics year round regarding income and other issues. The bottom line is that, if you want to benefit from that info, participate. If you want more proliberty State Reps, send in the form.