[XeTeX] wrong glyphs with FreeSerif Italic

[Julian Bradfield]

This is probably FA, but I haven't found it by searching...

I'm a first-time user of xelatex (but 30-year user of TeX in general),
and have used it to typeset a linguistic article with Charis SIL. I
then wanted to switch to GNU Freefont, and encountered the weird
symptom that all the glyphs are displaced by two codepoints in the
Italic version. 
Here's a minimal example:

\documentclass{article}
\usepackage{mathspec}
\setallmainfonts(Digits,Latin,Greek,Special)[Mapping=tex-text,Fractions=Off]{FreeSerif}
\begin{document}
ABCabc \it ABCabc
\end{document}


On processing, the PDF shows ABCabd CDEcde; the right character
metrics appear to have been used, but the glyphs are wrong.

My xelatex version is
This is XeTeX, Version 3.1415926-2.3-0.9997.5 (TeX Live 2011) (format=xelatex 20
12.11.27)
and the Freefont is the release of 20120503 (in either otf or ttf).