Get 'er done early and mail it back by April 1, otherwise you can expect a visit from census enumerators. And that is where is starts getting expensive for the U.S. Census Bureau. Enumerators are paid and reimbursed for mileage, so the cost of getting the count at one home can escalate quickly, especially if enumerators have to make multiple visits.
The decennial census is required by the Constitution and participation is required by law. Your answers containing personally identifiable information are 100% confidential and cannot be given to other government agencies... in fact, the Attorney General ruled this week that Census confidentiality trumps the Patriot Act. Census enumerators will have an ID badge with a toll free number you can call to confirm their identity and they will not ask for Social Security, bank, or credit card numbers.
More info about the Census, how the count will be conducted at places like college dorms and nursing homes, and toll free helpline numbers can be found in this helpful article in The Roanoke Times.