It might be too early for this. WebKit implemented it fairly recently, there's no hint of support in any mobile WebKit at all, Opera just released support for it, and Gecko's implementation is still in alpha. IE? Hah.
But as far as I can tell, no, there's no polyfill for the new flexbox. Flexie supports the old flexbox, and has a ticket open to support the new syntax... maybe you could give them a hand?
You could always use the old flexbox, I suppose, but then you're obsolete out of the gate. Sucky situation.