Jesper,
On load, Office 2007 SP2 will simply ignore the unsupported elements and attributes in ODF files. We do not attempt to round trip unsupported elements and attributes, they will be removed from the ODF file if you resave it using Office 2007 SP2. This is consistent with our implementation principles and our desire to provide predictable behavior. We considered trying to roundtrip elements and attributes that we do not understand or support, but we found if we did this that we could not be sure the resulting files were internally consistent and conformant ODF files.
As an aside, there are some cases where we write elements or attributes on save that we do not support on load, for the sake of better interoperability with other applications that use ODF. Those cases are described in the implementer notes as well.