Sounds bogus - - pitch it.
It's legit, or at least, it really comes from Big Brother...
http://en.wikipedia.org/wiki/American_Community_SurveyQuoteth the article:
"To date, no person has ever been charged with a crime for refusing to answer the ACS survey, which several U.S. Representatives have challenged as unauthorized by the census act and violative of the Right to Financial Privacy Act. The Department of Commerce states that it is "not an enforcement agency."
Personally, I'd toss it, or if I was feeling specially mischievous, print a copy of the Constitution, stuff it in the envelope and send it back... Of course, that'll get ya' on a watch list as a potential terrorist, but oh well, I'm sure most of us are already on one of those lists by now anyway...