This isn't as big of a deal as some make it out to be. Prior to the 2010 census, citizenship was asked.
Question 13 of the 2000 census form specifically asked the following.:
Is this person a CITIZEN of the United States?
-Yes, born in the United States—>Skip to 15a
-Yes, born in Puerto Rico, Guam, the U.S. Virgin Islands, or Northern Marianas
-Yes, born abroad of American parent or parents
-Yes, a U.S. citizen by naturalization
-No, not a citizen of the United States
This question, in some form or fashion, has been on the census since 1850.