a document in OOXML format could be really dangerous, according to IBM's Bob Sutor.
Sutor, who is Open Source and Standards Vice President at Big Blue, wrote in his blog that saving documents in OOXML format right now is probably about the riskiest thing you can do if you are concerned with long term interoperability.
He said that the official ECMA OOXML that was submitted to ISO is not what Microsoft implements in Office 2007 so it does not work there. He said that nobody has the vaguest idea what OOXML will look like in February or even whether it will be in any sort of stable condition by the end of March.
As a result, the OOXML format that you are saving files in right now could be dead and will be replaced, unless Microsoft decides it won’t bother implementing what comes out of the ISO process.