The agency responsible for conducting the census in England and Wales (the Office of National Statistics) has just completed a 'dry run' for the 2011 census..

The 2007 Test included approximately 100,000 households from within the five local authority areas of Bath and North East Somerset, Camden, Carmarthenshire, Liverpool and Stoke-on-Trent. Its main purpose was to assess the feasibility of the major innovations in the proposed 2011 Census design.
The ONS website carries much useful information about the census and in particular, there are a number of factsheets about the design and purpose of a census which you could do well to look at..
