A new development:
Now you can latex equations in Thunderbird
http://www.qscitech.info/blog-entries/LaTeX-in-your-email-with-Thunderbird.html
https://addons.mozilla.org/en-US/thunderbird/addon/6247
Hi,
I just saw this forum thread and was looking for an email client with such capability also.
But this is what I do in the meantime:
I uses LatexRender to create equations in .png/.gif files (I feel that .png files are sharper in detail) and insert them as pictures in my HTML emails...