Well the breaking news at the moment is that when you install Microsoft Office 2003 Service Pack 3 (SP3), it blocks off a number of file formats. You can read the Microsoft Technote on it here.
In particular, Microsoft is preventing office 2003 from saving (and possibly reading - I can't tell yet) a heap of Lotus and Quattro files (including the various flavours of .wk files, which will upset a lot of people who export from Lotus Notes).
They're also blocking off versions of Powerpoint older than Powerpoint 97.
The official excuse is that these file formats are less secure but I think it has more to do with making sure that people upgrade. How long will it be before they declare Office 97 unsafe in order to force a move to Office 2007 and XML?
All this seems to do is put even more weight behind the need for common file interchange standards that are owned and administered by the International Standards Organisation (ISO) rather than by a commercial entity like Microsoft.