For example, in the Android app:
Same post in a (mobile) browser:
MathJax is enabled in application as of version 1.0.13, coming out later tonight. It's only supported in a limited selection of places:
MathJax is not rendered in question titles anywhere. That means in the feed, search results, and even when looking at an individual question.
This is something we've talked about a bit before and decided to delay figuring out what to actually do until after our initial release.
To give more explanation here:
Since everything in the app other than the question/answer bodies are native code rather than webviews, this is going to take us a while to try and figure out. At the moment we're putting it on the backburner.