“The Office specifications being made available now include all of the binary file formats used in releases of the desktop applications suite from Office 97 through the current Office 2007. The native file format in Office 2007 is Office Open XML, not a binary format. But Office 2007 can still save files in binary formats that are basically identical to the ones used in Office 2003.

New Zealand’s PCWorld has details

Microsoft’s current publicly available MS Office file format documents  can be found here