See <http://www.w3.org/TR/DOM-Level-3-XPath/xpath.html#NamespaceNodes>. From my testing, it seems that Firefox 2.0 also doesn't support namespace nodes, while Opera's support appears buggy.
Created attachment 12825 [details] test case
Comment on attachment 12825 [details] test case Test looks OK to me. To turn it into a layout test it would be better if it was a simple "pass/fail". Maybe you're looking for someone who has read the XPath specification more carefully to tell you if you did something wrong -- if so, sorry for marking this reviewed.
Comment on attachment 12825 [details] test case Oops - sorry! All I meant was to attach a test to the bug; must have marked assigned and r? out of habit :-/
This is almost certainly a WONTFIX, namespace nodes are pure evil.
And let's just do that.