[XeTeX] problem getting small caps

[maxwell]

I'm having trouble getting small caps to work with a font which appears to
support small caps.  Here's the minimal file:
----------
\documentclass[12pt,letterpaper]{report}
\usepackage{fontspec}
\setmainfont{Charis SIL}
\begin{document}
This might be a document.
\textsc{This is some text in small caps.}
{\scshape This is some text in small caps.}
{\sc This is some text in small caps.}
This is some text which is not in small caps.
\end{document}
----------
You'll notice I've tried three different ways of getting small caps to
show up.  Nevertheless, the PDF shows no small caps, unless I comment out
the \setmainfont{Charis SIL} line, in which case all three work.  There are
no obvious messages in the .log file.

We've had several versions of the Charis SIL font installed, and the older
versions probably did not support small caps.  So my guess was that somehow
fontspec is picking out an old font file.  However, when I look at
/etc/fonts/fonts.conf and /etc/fonts/local.conf, and use 'find' to locate
the copies of Charis SIL regular in the directories specified there, and
then run otfinfo -f on those files, it always returns
   ...
   smcp    Small Capitals
   ...

And otfinfo -a reports "Charis SIL", so I think the name I'm using in the
\setmainfont command is correct.

Suggestions?  Do I need to explicitly tell \setmainfont that Charis SIL
supports small caps??  

   Mike Maxwell