User Manishearth has kindly written a script that will insert $\LaTeX$ dollar symbols automatically in the editor window on Stack Exchange sites that support MathJax. He has extended the script to work on Quant.SE. Feel free to check it out.
Note to users who installed it before 5/5/12 (3:51 PM UTC):
The script no longer needs to be updated (most updates include the addition of more sites/buttons and tweaks like tooltips). It auto-fetches the updates now.
I suggest you uninstall/reinstall it to run this one-time update
How to uninstall:
Then re-install normally (click here)