Can Word open a .odt file?

Ok, now we're talking about ECMA and not ISO... That MS was caught "off guard" seems rather unlikely to me, since MS is one of the key members of ECMA.

And again: Code for import/export of ODF should be rather platform independent in itself. Which OS-specific syscalls would one need to interpret an XML-datastream?

The reason why Office for Mac didn't get ODT support was probably just a matter of the timing of the announcement by EMCA regarding OOXML. 

Microsoft was betting ODT was going to be the open standard, so they spent a ton of time and money incorporating ODT into the Office 2007 product fully expecting ODT to be chosen as the open standard. When ECMA announced OOXML as the open standard, Microsoft was caught off guard just like the open source community was. Since Microsoft had the ODT stuff done for Windows Office 2007 they kept the ODT support they had built and then switched to implementing OOXML support for 2007 as the default.

As you might guess from the product name and delivery dates, Office 2008 for Mac was about a year behind the 2007 development cycle. By the time Microsoft started working on 2008 for Mac they knew that OOXML, not ODT, was chosen as the open standard. Since they were now focused on OOXML instead of ODT, they decided to skip ODT altogether for the Mac and concentrate on OOXML. I suppose the thinking was "why bother with ODT now that it is on the way out?" While Microsoft had the PC ODT code, implementing it on the Mac would have been additional overhead and testing on a technology that was no longer the official standard.

So, why is there no support in 2011, although the code to read and write ODF is obviously available? It just makes no sense. Please don't tell me, that the import and export filters for ODF can't be compiled for MacOS or that the architecture of Office for Mac is so different from the Windows version, that the code is incompatible... As a software developer myself I just cannot find any technical reason (assuming that the developers at MS are skilled enough - which I'm pretty sure they are).

And I'll ask again: why replace a modern ISO standardized file format, but keep support for much older formats? Again: makes no sense.

To tell you something about my background: the company I work for is Linux/Mac-based. We are no Windows-haters, but Linux+Mac works perfectly for our core business and in fact many of the software products we're using aren't even available - or at least not native - for Windows! Consequently we are heavily using OpenOffice/Libreoffice and therefore ODF for our internal documents.

Since we are growing, our finance/legal department is growing too. This is the only part of our company where MS enters the picture: most external documents in this area arrive as OOXML 'transitional' - much because of the aforementioned issue of MS promoting a new 'standard' before being able to write files that actually adhere to it. And - not surprisingly - OO/LO has some difficulties with these, because 'transitional' is all about using legacy stuff, where no public documentation is available (which was the main reason for ISO to split the standard into 'transitional' and 'strict').

So the most simple solution for us would be to buy some copies of Office 2011 for our Macs. The only problem is, that 99% of our internal documents are in ODF, which cannot be opened in Office 2011. Of course, there is LO for Mac, which we can use in parallel and convert the documents - but still it's an unnecessary nuisance.

And yes, I know that Windows can be integrated into a unix-based LDAP/NFS-environment; but it just doesn't work as seamlessly as the Macs do. OTOH buying Windows licenses does not really make sense either, because we actually would not need it for anything else than running a version of MS Office that can read ODF. That would be a sledgehammer to crack a nut.

Yes, Office 2013 is at last able to write "strict" Files. Office 2010 and 2011 (which is the latest version for Mac) are not

According to whom? Office 2011 was promoted as being the first software to meet the strict standard, followed shortly by Office 2010 service pack 2.

- and yet the support for ODF was removed in these versions!

ODF support was never included in Office 2011 for Mac. A good level of ODF support was included with Office 2010 and was carried forward with Office 2013. ODF support was never removed from any Microsoft Office product.

The only reason for this is - once again - that MS is trying to push it's own standards... Or can you think of any other motive to remove support for an ISO-approved, open, and actively developed standard, while still maintaining support for much older, proprietary formats?

Your assertion that ODF support was removed from Microsoft Office is false.

As I already stated, although ODF is an open file format that is backed by an open standard, the ODF format was replaced by OOXML. There's not much point in adding support for older technologies when the world has by and large adopted its replacement. Beginning with Office 2007 Microsoft adopted the Office Open XML standard as the default file format for Word, Excel and PowerPoint replacing proprietary DOC XLS and PPT file formats with the current open format.