What is interoperability, really?
Well, when it comes to document formats, some people seems to think that interoperability is the ability to transform one format to another. That high-fidelity interoperability can only be achieved when it is possible to perform a complete translation/conversion of format X to format Y.
The basic problem for this premis is that if you were able to do this conversion, it would be the same as being able to make a 1-1 mapping between the functionality and features of format X and format Y (and vice versa). However - this effectively means that format X is actually just a permutation of format Y ... making format X and format Y the same format (pick up your favorite book on mathematical topology to see the details).
When it comes to ODF and OOXML, the case is pretty clear - the two formats are not the same. Sure - they can both define bold text, but there are quite a few differences between the formats. A list of some of them can be found at the ODF-Converter website. I think that the list is the best argument for not being able to do a complete conversion of ODF to OOXML (and back). This was also one of the conclusions of the Frauenhofer/DIN-work in Germany, where they concluded that a full 1-1 mapping between the two formats could not be done.
The key question here is: Is interoperability diminshed by this fact?
If you ask Rob's posse, they will almost certainly say "Yes". They will say something like "Microsoft chose not to make OOXML interoperable with the existing ISO-standard ODF and therefore OOXML is a blow to interoperability".
If you ask me, I will say "No". I will say no because the term "interoperability" has been hijacked by the anti-OOXML-lobby in much the same way the SVG-namespace was hijacked by ODF TC. I will say "No" because interoperability means something radically different. The meaning is not rocket sciency, really ... and usually most people agree with the basis definition of interoperability. A few of those are:
Computer Dictionaly online:
The ability of software and hardware on multiple machines from multiple vendors to communicate.
the ability of two or more systems or components to exchange information and to use the information that has been exchanged
US e-Government Act of 2002:
ability of different operating and software systems, applications, and
services to communicate and exchange data in an accurate, effective,
and consistent manner.
If you also look at the enormous list from Google you will see, that none of the definitions talk about the ability to convert formats. Instead they talk about communication between machines, platforms and networks. This is very close to my definition of interoperability when it comes to document formats.
The interoperability gained by using a specific document format is based on the possibility of implementing the format on any kind of platform, in any kind of software using any kind of operatingsystem. It is based on how well and consice and clear the language of the specification of the format is and it depends of howwell thought out the specification is.
It has nothing, nothing, nothing to do with the possibility of converting the format to any other format.