Here's Ancestry's response of 8th October 2009 (not very helpful I'm afraid):-
"The 1911 Census is currently only available via a pay-per-view agreement on the 1911 Census website.
Ancestry.co.uk is keen to offer members the richest and most diverse range of family history content available and has begun to discuss with The National Archives the possibility of securing a license to offer our own online access to the 1911 Census in the near future.
As part of our ongoing commitment to providing our members with the richest and most diverse set of historical records, in a world first Ancestry.co.uk will shortly launch online the collection of the
London Metropolitan Archives, which predates civil registration in 1837 and date as far back as the 1500s."