Office XML compatibility
Office 2007 has also been completed, and the compatibility pack is now available to allow earlier versions (back to 2000) to read the new file formats.
Though as Office Watch points out, while the Word one works well in both directions, it appears that the Excel and Powerpoint don’t have full conversion, just viewers.
What a copout. If they can do it for Word, they can do it for the others. Mind you given the plethora of information about the new formats I wouldn’t be surprised if a third party writes a proper conversion tool to fill the gap.