We're on the same page as the Mathematica people here, and I don't think we should worry about any winner-take-all, king-of-the-hill story here. IPython Notebook has a Mathematica flavor and that's fine, so does Sage. We're in a synergy relationship.
That's especially true in the space of my workplace, where a lot of the brain cycles have been committed to Hilbert, software that brings in Mathematica over the server to a browser-based client. Others of us teach Python and other executable notations. Per your integrating vision, it's all one domain.
http://www.makingmath.com/ (same group as O'Reilly School)