Shred it.
I tore the tag off my mattress once.
This time around they are going to take a GPS coordinate on your doorstep:
http://www.census.gov/procur/www/fdca/april22-posting.html
Then, they'll be able to do all sorts of things!
If you have a little time to kill, you can really have some fun making up answers to the census questions. The only question I ever answer accurately is "number of people who live at this address".
Yes, your response to this survey is required by law
It says "your response", not your 'truthfull and complete answers'. Send it back. Make sure they know who it is from.
You've responded.
Just send the form in with the questions unanswered, by sending in the form you are replying.