DOCBOOK: xref into another document?

I'm using docbook-website to build my site and I'd like to create xrefs 
among pages. There's nothing special about website, so it comes down to 
creating xrefs into other documents.

Of course, I can (and do) fake the result with ulink, but this is a 
recipe for inconsistent references.

Michael

-- 
Michael Schuerig                  If at first you don't succeed...
mailto:           try, try again.
http://www.schuerig.de/michael/   --Jerome Morrow, "Gattaca"