Office 12 Buzzsite
The Office 12 buzzsite is now up and running. It's the best place to sign up for future updates on product news, and currently has some great resources around the XML formats. The two whitepapers that I've already linked to are there, as well as Q&A and information guide. Between my blog and that site, you should be pretty well covered for new around the next version, and especially around the new formats.