[XeTeX] Problem getting ffi ligature

[Dr J.D. Billett]

I've been trying to use TeX Gyre Termes as a Times New Roman  
substitute in typesetting a book with XeLaTeX (I've finally got the  
darned thing to compile after wading through a lot of errors that  
weren't a problem under pdflatex).  I am having trouble persuading  
XeLaTeX to give me the "ffi" ligature.  I am certain that it exists  
in Termes (at least, it's visible in Font Book).  But in its place I  
get an ff ligature followed by a regular i, as in the following  
minimal example:

\documentclass[11pt]{memoir}
\usepackage{fontspec}
\setmainfont[Mapping=tex-text,Ligatures={Common}]{TeX Gyre Termes}

\begin{document}

\noindent fi fl ff ffi \\
\textit{fi fl ff ffi}

\end{document}

Suggestions for how to fix this would be most welcome.  My book makes  
constant reference to the "divine oFFIce", so this is important!

Jesse Billett