there are new special features that are only available in 2008 version. apparently. these new special features will be for sure addressed in the update.
ie. formating and text coding etc, that office 04 won't be able to understand.
especially in excel, they have alot of new formating palets and what not that isn't compatible with 04.
for .ppt. i'm not to sure. but i'm sure there are new features in .ppt that office 04 can not open.
as well there are new fonts i think.. maybe that too..