Two InfoWorld articles touching on Office 12 XML.
This one mentions PowerPoint and the overall big picture.
This one looks more at XML, interoperability, and backward compatibility. The gem was snuck into the last paragraph:
Versions of Office 2000, Office XP, and Office 2003 are compatible with the new software, which can be downloaded by users of those versions for free. After the software is downloaded, users can open, edit, and save new files with the new format from within those earlier versions of Office, company officials said.
That’s a relief.