[jcl]

If the big barrier to acceptance is intranets -- where the company controls the web server itself -- then an additional solution is available: Rather than requiring all new web pages to contain a new "OK for IE 8" flag, simply tag the old pages with a new "Not OK for IE 8" flag, to trigger legacy rendering. I imagine you wouldn't even need to edit the webpages -- just add some module to the web server that automatically rewrites the header.

...There must be a good reason why this would never work, because I don't recall hearing it proposed before.

[mdemare]

The good reason why it'll never work is that it has to be done for bilions of pages.