The Library and Archives Canada made the controversial and unpopular decision to have Ancestry digitize the 1921 census, in return providing them with 4 years of exclusive distribution. While free to Canadians, it did require the creation of an Ancestry.ca account. (The unpopularity of that decision may be why the news release on the LAC website wasn't pushed to the press - I can't even find a recent story doing a search of news sources using Google.)
Well, the period of exclusivity is now over!
The 1921 Census is now up on the LAC website.
Details are here: http://www.bac-lac.gc.ca/eng/census/1921/Pages/introduction.aspx
Search here: http://www.bac-lac.gc.ca/eng/census/1921/Pages/search.aspx
Fellow Canadians, get sourcing!
As an addendum, it seems that the Canadian website, Automated Genealogy, already has a transcription project underway.