Paul is right in that many Zope 3 developers, and many Python developers in general, don't give XML a high priority. I wrote about some of the possible attitudes surrounding this before.
I am involved in Zope 3 development, and I care deeply about XML support, and have been thinking it over for, well, years now. In Silva, we have been adding more XML support with every release, and are going to release more soon, so we now have quite an extensive background in these matters too. We'll be looking into porting this functionality onto the Zope 3 platform.
Infrae and Nuxeo recently announced that we will be working together on Zope 3 development. XML support will certainly be a part of this cooperation.
What shape this XML support should take in Zope 3 needs to be debated. It might be in the core, it might make more sense as an extension. It might be a combination of both. One way or another, Infrae be helping to develop it, and our aim will be to make the functionality attractive enough so that anyone who wants to do serious application development with Zope 3 will be tempted into using it.