It would be best to do this somewhere that supports MathJax (
http://www.mathjax.org/). HN does not, which could make it awkward to discuss math in an HN thread.
There are three reasonable ways to add MathJax support to a site.
1. Get the site owner to do it. The site owner simply has to toss in a script tag that loads MathJax from the MathJax CDN and sets some configuration options. Someone would need to convince PG that this is worthwhile. (I've done some timing tests, and MathJax doesn't seen to cause any noticeable performance problems when used on pages that contain no math. Any decent browser caches the script so load time of the script is not a problem).
2. The user can use a Greasemonkey script to load it, for browses that support Greasemonkey scripts. Here's an example that loads MathJax on Reddit: http://userscripts.org/scripts/review/108770. This could trivially be modified to work on HN instead of Reddit.
3. Safari doesn't support Greasemonkey scripts. There is a Safari plugin to add that functionality, but it does not work very well. I wrote a Safari extension to load MathJax (or Tex The World, discussed below) on Reddit: https://github.com/tzs/Reddit-Math-Display-for-Safari. This could easily be modified to work on HN instead of Reddit.
On Reddit, in /r/math, the convention is to use LaTeX delimited by [; and ;] to mark math, and to use a script called Tex The World to render it (http://thewe.net/tex/). That script finds the math, and sends it off to CodeCogs.com where it is rendered and an image is returned for display. This occasionally causes problems due to the load on CodeCogs.
To work around this, someone has a hacked version of the that script (link available in the sidebar on /r/math) that uses the Google Chart API to render small equations and does some kind of caching.
I'd recommend NOT using either of these. The author of TeX The World stopped work on it quite a while ago. There's no licensing information on the site so it is not clear if anyone else can take it over, and by using an external service to render the math it is fragile.
MathJax seems to be the best approach. That's what they are using at mathoverlow and math.stackexchange.com. Note that if MathJax is loaded by the page itself, rather than by an extension, then it works great on iOS and most other major mobile web devices, in addition to pretty much all major desktop browsers, without the need for the end user to do anything special.