A coworker of mine, just reminded me that for those folks interested in using the new Open XML formats but don't want to upgrade to Office 2007 Beta 2, we also released a public preview of the compatibility packs (thanks Jon!).
This is one of the other really important points I've tried to get across over the past year. These new formats aren't just for people who have Office 2007. You don't need to upgrade in order to use them. You can use these compatibility packs with your existing copies of Office, and you'll be able to read and write the Open XML formats.