Update: Stack Overflow Enterprise is now available for internal use.
I can't upvote yet, but +1 for the enterprise edition of the Stack Overflow engine. Most companies already have a portal/sharepoint site and they have their own user communities. Any company with more than 2 geographical locations and 100 employees should have its own internal expert community/mailing list IMHO, and most have a formal/informal version anyway.
Rather than everyone reinventing the wheel with Exchange, SharePoint, wiki, etc., etc. a Stack Overflow engine on a 3 year release cycle (SharePoint-like) would be a boon for most companies, and they would be happy to pay for it.
In fact, once you release a stable enterprise edition, where an SE can install and configure it easily on their server and link to it from their portal, you will be surprised at how many other companies will jump on board.
SharePoint is one of the quickest selling products of Micrsoft, not because it's technologically superior or ground breaking, but it solves an easy-to-understand business problem, in an easy to understand/demonstrate way (the front end anyway, the back end maintenance/installation is another story).
And it's not just for programmers either, I can easily imagine this sales pitch to management - "Look, every time we have to make a sales pitch for product/technology X we send emails around asking for slide decks, case studies, customer testimonials, etc. which just clutters up everyones email boxes. SharePoint sites lacks specific solutions and individualised answers. But with this engine, we can easily create a site where I can ask a question like "Has anyone sold product/tech x to company y", "Does anyone have a contact in team a of company b" and we quickly get answers without spamming everyone on the list as well as identify experts/high achievers in our company that we can recognise and reward. And the geeks we have in the basement can ask their own geeky questions as well and we can just ignore them."