Centering chapter titles with fop

Hello,

I'm using docbook 4.4, docbook-xsl 1.69.1 and fop 0.25.

I try to get the preface, chapter and appendix titles centered, while  
section titles are left-margined, but cannot. It used to work with  
docbook-xsl 1.68.

Is there something I missed?

Cheers,
Mich鋩e
<http://micmacfr.homeunix.org>