Erwin, it does not necessarily have to be a big problem for OpenOffice.org or other FOSS wordprocessing/spreadsheet/presentation software to support the Open XML document format. I do not say, it will be trivial (over 4000 pages of specifications is quite a long reading for sure), but the fact that M'soft is dumping into the format their internal data representation means that one would not start from scratch implementing it. The binary MS Office formats -- that FOSS applications support quite well -- are also in a big extent the dump of the Office's internal data representation and it is quite unlikely that M'soft is undertaking any significant rewriting of the MS Office's internals.
So, IMHO, the solution is to fight successfully the temptation that people have to speak without thinking about XSLT when they hear about XML to XML conversion, to look beyond the container at the content and realize that the code in the WW8 filters might be to a big extent reusable for supporting the Open XML .docx file-format at least.
