You need local-name():
http://www.w3.org/TR/xpath#function-local-name
To crib from http://web.archive.org/web/20100810142303/http://jcooney.net:80/archive/2005/08/09/6517.aspx:
<foo xmlns="urn:foo">
<bar>
<asdf/>
</bar>
</foo>
This expression will match the “bar” element:
//*[local-name()='bar']
This one won’t:
//bar