As far as I know, it is only needed in the <html> tag, but not elsewhere. From the XHTML 1.1 spec:
The start tag of the root element of the document MUST explicitly contain an xmlns declaration for the XHTML namespace. The namespace URI for XHTML is defined to be http://www.w3.org/1999/xhtml.
and of course, the "root element" is the <html> tag.