> >>> Tip: Functions are often called with the fn: prefix,
> >>> such as fn:string(). However, since fn: is the default
> >>> prefix of the namespace, the function names do not
> >>> need to be prefixed when called.

http://www.w3.org/TR/xquery-operators/

The XPath 2.0 documentation on function and operators explains some of
the details of the namespaces in the first section.  Don't ask me to
explain it though ;).  I'll leave that to some people who've done more
than I have with XSLT 2.0.  (I've poked and prodded it but haven't
used it in any large-scale project yet.)

Jon Gorman