Upon reflection, Emilio is 100% correct: We need to sound the alarm bells. Here's a link again to Emilio's well-thought-out answer on the main meta site explaining this, which is probably the best place to start our campaign. In short:
As currently envisaged, the only way to preview MathJax before publishing a post will be by toggling out of markdown mode and into the new rich text editor.
This is unacceptable, because it means that instead of a real MathJax preview, we will be using whatever pale imitation of a MathJax preview the developers at Stack Exchange manage to shoehorn into their rich text editor in their spare time. For instance, it's likely there will be no way to preview the rendering of user-defined macros.
The only solution I see is to convince Stack Exchange to include some form of dedicated preview for markdown mode which is not reliant on rich text mode, and which fully supports MathJax. Given that this is a less demanding requirement than what currently exists, I pray that our overlords will find it feasible to do something like this.
To my mind, the most important thing is not whether the preview is side-by-side, stacked vertically as at present, exists as a separate view, even a third "mode" in addition to markdown and rich text, whatever. The most important thing is that it exists, is independent from the rich text editor and fully supports MathJax. But we should start thinking about details like this -- if all goes well, Stack Exchange ought to be soliciting our input on such matters as we move forward.
I've also left an answer at the main meta site discussing this in more detail. It's a chore to look through all the answers over there, but it appears that others have also raised concerns about MathJax integration. Emilio also points out there there are several sites which also rely on other formatting plugins, such as chess, go, furigana and music notation, which would suffer much like MathJax users in the currently-envisaged system. We need to band together and clearly articulate our needs. As Emilio points out, MO's unique arrangement with Stack Exchange gives us some extra leverage compared to many other sites. For this reason, and because we would among the sites most severely affected by theses changes, it is incumbent upon us to take a particularly active role in this effort.
A bit long for a comment:
Thanks, Emilio, for bringing this up here! I already flew off the handle in the comments to Emilio's answer at the main meta question under the impression (which I now believe to be mistaken) that the proposed changes would force one to use rich text with no option to revert to markdown. I now understand what they're proposing would include the option to revert to markdown, but that there would be no live preview in the markdown mode.
This is not as catastrophic as I first thought, but it would mean you're tying a hand behind your back. In order to catch minor errors you'd have to either publish and see the render and then make a series of public edits, or else toggle to rich text and cross your fingers that the rich text representation is adequate to catch stuff.
As pointed out over there, the preview is actually in some sense an upgrade over many latex editors because of the automatic and constant refresh. I suppose I don't need the refresh to be so frequent, and I don't need it to be automatic, but it sure is nice. If it's not going to be there, I would prefer to have a "compile" button rather than "toggling to rich text mode". The latter feels more clunky -- every time I toggle, the refresh is going to be loading all sorts of rich text functionality which is irrelevant to me, in addition to rendering the mathjax that I actually need to see.