I've speculated on this a number of times, but now an article from CNet is talking about Microsoft's moves to integrate "rights management" technology into the Office 2003 product.

Microsoft is selling it as an enhancement that allows businesses to determine who will be able to read confidential data, however CNet and others have caught on to another issue, which is that files written using Microsoft's DRM can only be read with the MS DRM system in place. This means that other platforms, such as the Macintosh, may have problems reading these files. Further, even programs that can read and write Microsoft Office format files (Excel, Word, PowerPoint, etc.) may not be able to do that on versions that use the DRM system.