First, take a look at Shapado, an open source clone of StackExchange.
While the site is far from mature, it is fast growing and active (if there are other similar clones, this question can refer to them as well). You can get a free, hosted site "in 8 seconds", and there are already some features there that I would like to see in StackExchange.
In light of this, does the answer to the question about open sourcing Stack Overflow change? I think that if Stack Overflow doesn't open up soon, Shapado and its clones might build a large enough development community behind it, and Stack Overflow might never catch up. If Stack Overflow opens up now, yes, it will cut down costs. But it will make the Shapado effort almost obsolete, because (if it's done right) devs will prefer to branch off Stack Overflow instead of Shapado.
Is it better open source soon or lose the battle later?
I believe this question should be asked, because open source clones have a decent chance of overtaking Stack Exchange (even if not Stack Overflow), and the business goals and code bases of the two projects are (very?) similar. If StackExchange will be at risk of being overrun by a clone, won't this be reason enough to open source it (and Stack Overflow along with it)?