I have just finished the first cut of adding proper html rendering to tiggit mail using the built-in browser object. You can see a HTML emails on the Storm Simulator below. (Thanks to Apple for the email advert to test with.)
It is by no means trivial to pass an HTML string to the browser and then get it to render it correctly. It feels like I wrote an entire browser this afternoon! My big worry now, is that it will not be fast enough, especially for those messages that do not come in both plain text and html.
HTML rendering will be included in version 1.1 preview 4 that will be posted in the next day or two, all being well.