Is the concern for doing it online versus via the mail a concern about the security of the actual data, or is it a concern about the security of the procees to prevent fraud.
Personally I find the entire idea and process of the census to be completely ridiculous. If the amount of information collected by cities and towns could easily be standardized through an electronic means, then there would be no need for a census like this to be done every 10 years.
The number of various registries in this country should allow for an easy method of tracking people close enough to the realistic numbers.
Births, Deaths, Drivers licenses, deeds, water bills, electricity bills, tax returns, and dozens of others should be able to take care of all this if the data is properly organized in the first place.
The waste in terms of information collection and storage is just plain ridiculous, and the lack of departments within the same government not sharing, or refusing to share information with other departments only makes matters worse.