Well you cannot do your tax delaration for 2010 in 2010, you always do this in the following year, because you only know your income at the end of the previous year. So how would to a tax declaration for 2010 already in 2010 without knowing all the income till 31.12.2010?
The second question depends on what kind of income as well on the declaration duties. Each nation handles it different. Probably you have to make a tax declaration, but with earnings of 2000 you will not have to pay taxes.
The census bureau needs and wants usually to know from all citizens the income, because it provides necessary data for politics and fiscal policy.