Back-door lock-in?
In light of the fact that ONLY Office 2000-and-up can officially read OOXML files at this time (and THAT requires a conversion add-on for Office 2000/XP/2003), I see this so-called "security" move as a net negative-- an attempt to lock users into MS software. Sure, there are stand-alone converters that will convert back from DOCX to DOC formats, but how many end-users will know this, or be in an environment that allows such software to be installed?
We can only HOPE that OOXML crashes-and-burns, as it deserves to. The fact that most professional journals won't accept OOXML file formats is telling.
-- Mikey