[XeTeX] fontspec doesn't like authors with accents...

[Fabrice Nauze]

Hello everyone,
I'm wondering whether someone would know a trick to solve my problem:
Say you have a bib file with an entry 'test' which author's name  
contains accents as follows,

@book{test,
	Author = {M\"{o}\c{s}{\i}{\"o}},
	Booktitle = {Accents in Author},
	Publisher = {XeLaTeX},
	Title = {Accents in author},
	Year = {2006}}

If you cite this entry \cite{test} using a bib-style returning the  
name of the author in brackets (as astron or kluwer styles), i.e.:

\documentclass{article}
%% \usepackage{harvard}
\usepackage{fontspec}
\begin{document}
\cite{test}
%% \citeasnoun{test}
\bibliography{test}
\bibliographystyle{astron} %% kluwer style with harvard package to  
use \citeasnoun
\end{document}

you obtain (with XeLaTeX) either a message of this kind:

!undefined control sequence
\UseTextAccent ...up \@firsttofone \let \@curr at enc
	\cf at encoding \@use at tex@en...
l.5 \cite{test}

or a pdf without the reference in the text, i.e. with '(?)' instead,  
although the author's name is spelled correctly in the list of  
references!!!

Some things I've tried:
1) LaTeX works fine with those accents.
2) If fontspec is not loaded the formatting can be forced with  
XeLaTeX to obtain the desired result (but I get an '!illegal  
parameter' error/warning).
3) The command \citeasnoun (with harvard package and kluwer style)  
works fine with fontspec and XeLaTeX, i.e. I obtain 'Mosio  
(2006)' (with the correct accents) in the document!!!

I guess that's it! thanks in advance for the help,
Fabrice