Ancestry.com will blow the government site out of the water on this one. They will index first and offer the 1940 for free to the public. Marketing genious, as once hooked, they will have to subscribe for additional census records and other documents that automatically appear as you search an ancestors census record.
When the US releases the 1940 census it will not be searchable by name for some time, which is far and away the most attractive way to search it, and ACOM is expected to create a name-searchable database. That should work well to their advantage as people who visit the archives first will be driven to their site. And I'm sure they'll make this widely known.
P.S. Sorry to hear about your serious-academic-historian wife. I know trophy wives are much more popular.
The National Records and Archives (NARA) will release images of the UNINDEXED 1940 Census on April 2, 2012. There is NO way to search these images by surname. There is a very complicated method described on the NARA website to determine the Enumeration District (ED) for your ancestor based on whether you have even the foggiest idea of where they were living in 1940, that might lead you to multiple possibilities. At that point you have to read every page of that ED to find your ancestor (some EDs are 150-200 pages long). Ancestry.com has an indexing project that will start on April 2, as does a triumvirate (findmypast.uk, familysearch.org, archives.com) that also has an major indexing effort starting April 2, using all volunteers - these indexed databases will be free forever. It's a race to see who finishes first, as well as which is more accurate (indexing is not easy).