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.
One possible solution is to identify as IE <= 7 to intranet sites (which can be designated via Group Policy) and IE >= 8 to non-intranet sites. Of course, this means admins have to properly define intranet/non-intranet, which means much rioting will ensue.
Poor IE team. You're getting screwed because management decided to rest on their laurels.