When I wrote the mimetex plugin I looked at a couple of different solutions... I didn't like the MathML stuff for three reasons (two of which are personal problems, not technical *grin*).
MathML support is poor: IE has a thridparty plugin to make it work, and even Firefox has to download Math fonts to use it...
I didn't see a nice way to input MathML into posts (a personal problem): I don't know the MathML structure and it looks like it would take a while to learn.
I don't know XHTML/MathML (a personal problem): I'd love to learn the XHTML and MathML specs, but I don't have the time.. And trying to do a plugin without understand those specs would have hurt.
I did look at some ideas, though:
MimeTex is pretty poor resolution.. eveyone who knows such things says that using latex and imagemagick will produce better results. An example is latexrender (http://www.mayer.dial.pipex.com/tex.htm#latexrender
), which actually has a number of plugins for various packages (including Wordpress). I ruled this out because it would require two executables to be called and I didn't want to have to learn/code conversion process. Latexrender does have a class file which might be useful.. it might be easy to modify the event_mimetex plugin to use it.
itex2mml looks very promising... http://pear.math.pitt.edu/mathzilla/itex2mml.html
I plan to come back to this solution this summer sometime... It is an executable which looks like it can parse html with itex syntax strings in it and convert the appropriate portions to MathML. I believe one could write a markup plugin for serendipity which would be a wrapper for this... if this is the case (and I understand XHTML to any degree, which I'm not sure on) you could send the appropriate headers using frontend_configure and dynamically do the markup... which should work in the preview, etc....
Like I said, I'm not sure on itex2mml since I don't know anything about xhtml.. But I do plan to look at it, since it offers a sane way to enter data directly to a post and still use MathML... And I think getting MathML as a commonly used standard makes tons of sense.
Hope that helped..