% -*- coding: utf-8-unix -*-
% format using xelatex
\documentclass{article}
%\usepackage[no-math]{fontspec}
\usepackage{fontspec}
\usepackage{xunicode,xltxtra}

%\defaultfontfeatures{Mapping=tex-text}
%\setromanfont{Latin Modern Roman} % I don't get the pi
\setromanfont{STIXGeneral} % this works for me
%\setromanfont{Charis SIL} % this works for me
%\setromanfont{Doulos SIL} % this works for me

\begin{document}
What is π?
\begin{equation}
x^2 + 2x = π
\end{equation}

%1/2 \quad 1/4 \\
%\addfontfeature{Fractions=On}
%1/2 \quad 1/4 \\
\end{document}