If you mean the XML displaytype (IE what you get by appeneding ;displaytype=xml to the URL of a node), I'm afraid that isn't going to happen anytime soon. The reason is that it would require pretty well completely reworking the code that does the XML displaytype. The code is designed to give out fields of the node, and the list of replies isn't one of those fields.
Sorry.
You can get the information you're looking for, however, from xml node thread; see What XML generators are currently available on PerlMonks?.
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