I dont think we need to get hysterical over lockheed- Google and microsoft can access far more interesting information about us if they wish! Good lord, even Mumsnet have access to more details about you than can be gleaned from the questionnaire! 
I just completed my training for census.
Just a few nuggets of information:
You need to complete all questions except the religion one.
Failure to complete every question will result in you having to fill in a new form from scratch - the collectors will not know what question you missed, only that you missed something. In this case you will be back on the collectors list and asked to complete a new form.
(If you fill in the form online, you will not be able to submit it unless you have completed all the information.)
You are legally obliged to complete the form truthfully - it has a statement at the back that says you have completed to the best of your knowledge.
Failure to complete the form could result in a £1000 fine and could also lead to a custodial sentence!!!
Prosecutions will happen - according to our trainer, people who dont do their forms are "low hanging fruit" for the CPS to make up their quotas.
Refusals will be recorded by the collectors - and will result in a different person coming back to discuss your refusal and the possible consequences. Details of your refusal will be recorded and may form part of the evidence against you if you end up in court.
If you have more than 6 people in your household, dont bother with the paper form, do it online.
on a seperate note, for the paranoid housewives of eg Pinner and Wimbledon, why would a multinational arms dealer care if you have central heating and two cars? You must have a very inflated opinion of yourself if you think the minutae of your life, your marital status, your date of birth and the number of bedrooms in your property have implications for national security! 