In Australia, and I would imagine all other countries, the census you submit is strictly confidential. The census collector, before being chosen for the job, has to declare that they have a safe place for them (such as a lockable safe) until they are given to a higher authority. The threat of a $1000 fine if you enter Jedi as your religion is not that great, unless, of course, they really DO collate your personal data in a way to connect it to you later, and that would open a whole new can of worms.
I suspect that 'Jedi' will be entered on many of our Australian census forms, in part to that darn email that keeps circulating, and also because our national youth radio station Triple J did a piece on it....rebellious teenagers and older larrikins will love the opportunity to mess with the system. My sister did such a thing last census - she payed homage to the 'George converts' episode of Seinfeld by putting Latvian Orthodox as her religion (an option if you have some strange hatred of Star Wars).
UPDATE: The Australian census went by on the 7th of August without any Jedi uproar, despite John Struik - head of the Australian Bureau of Statistics census section - warning that a $1000 fine was imposable for anyone giving false information. New Zealand residents also escaped fines on their March 6 census.
UK jokesters never had to worry - according to the census form itself "
'Completion of the Census form is compulsory under the Census Act 1920. If
you refuse to complete it, or give false information, you may be liable to a
fine. This liability does not apply to question 10 on religion." Phew!
In the UK it was reported that Jedi had actually become an official religion.This was because the Statistics Office released a list of religions found on census forms. Says The Register (theregister.co.uk) - "Coming very near the bottom of an official list of religions put out by the Statistics
Office, Jedi Knight is known by the code 896. Heathen comes in at 897. Followed
by Atheist at 898 and lastly None with 899. It's not exactly a ringing endorsement
though since the 800s come after every other religion, no matter how obscure, and
700 is used for all "other religions". The Statistics Office was keen to point out to the public that the inclusion of Jedi meant nothing...it was not a list of official religions, but simply a list of what people had entered on their census.