[XeTeX] Missing characters

[José Carlos Santos]

Hi all:

If I compile this file:

\documentclass{article}
\usepackage{fontspec}
\usepackage{xunicode}
\usepackage{xltxtra}
\begin{document}
$2×2±2$
\end{document}

all goes well; the result is a document which contains 2×2±2. However, 
if I add the line

\usepackage[utopia]{mathdesign}

before loading the fontspec package, my log file says:

Missing character: There is no × in font mdputr7t!
Missing character: There is no ± in font mdputr7t!

and all I get is "222". Is there some way of avoiding this (besides, of
course, replacing × and ± with \times and \pm respectively)?

Best regards,

José Carlos Santos