TechwareLabs has published a Guide to Saving Files in Office 2007
A quote from the article:
A Guide to Saving Files in Office 2007
Some of you who have new machines or who have upgraded to Office 2007 have already discovered that Office 2007 saves documents in a new format that is unreadable by those with the older Office 2003. If you want to save files created by the Office 2007 suite of programs in a compatible format then this short and simple guide is for you.
A Guide to Saving Files in Office 2007