One can use https://chart.googleapis.com/chart?cht=tx&chl= + LaTeX formula as URL for an image to display formulae, e.g. this (credit goes to Vortico for his comment on the Physics proposal at area51). It might be nice to add a separate button to the markdown editor or, even better, auto-expand $LATEX$ to
https://chart.googleapis.com/chart?cht=tx&chl=LATEX
edit Since google's TeX really looks ugly compared to alternatives stated in some answers (thanks to all who mentioned this) another API might be better suited