MS MBU will bring Microsoft Office XML Open Format to the Mac
The subject says all that's currently known. Unknown is for which versions of Office:mac the Microsoft Office XML Open Format will be available. Speculation is that Office 2004 will get an update for the format, Office v.X won't and the next version of Office for the Mac, due in 2006 according to insider reports, will be based on the format.
The new format is an important change in Microsoft's ways, because it is based on an open standard. For Mac users, it's important to stay compatible, as Windows users of Office will probably make use of the format anyway.