theborrower - the forms are not all sent out the same day, though the web site was live from 04/03/11 so I assume they expected some forms to have been received (the form has a unique 20 character code to identify the house, so you cannot just go to the website and fill census info in from scratch, sorry).
If you don't get one, then (from the form I didn't fill in, as I did mine online) the contact details are:
helpline 0300 020 11 30
website: www.census.gov.uk
Text relay 1800 103 000 201 160
Help also available in large print and Braille.