I think I like where you're going here, though my initial response was strongly negative. Here are my thoughts in the order they came to me:
* The search bar, when at the top of the page doesn't look like a text box; so I was confused. However, once the page scrolled down it was much clearer; possibly because it was separated from the rest of the page elements. Could that separation happen at the top of the page initially somehow?
* I don't think it's wholly appropriate to have the size of the search box be the same size or bigger than the page title.
* On the same line of thought; fundraising gets in trouble when we take up 'useless' space at the top of the page; could the top bar be made smaller?
* What happens with non content pages? In the interest of design unity, I'm unsure how you would apply the same design principle to say, Special:Preferences or Special pages which define their own tab groups like meta:CentralNotice.
* I really don't like the blue flashing edit button. Could that somehow be integrated into the floating header?
** Similarly, I don't think we should promote editing whilst hiding discussion. (In that the edit button follows you; but to discuss you have to move back up to the top of the page.)
* I feel that exploration, rather than search, is the primary method of navigation on the site, given that some large number of users come to the site via google. I may be atypical though in that I almost never use our search. That being said; my thought is that exploration tools like 'what links here', or the 'See also' section should be somehow inline to the search bar.
* I was pondering the idea of how one would use a floating footer to expose the 'Edit', 'Discuss', etc tools.
** Whilst I was doing that I realized I wasn't such a fan of the 'floating' effect of the search bar. Once again it seemed to be more about the search than the content -- I wondered what a 'sunken' search bar would do. If it would appear that the content was in front of search.