Naturally it is impossible to support what is still unknown. However, it would be possible to have some preliminary/beta support for the beta releases. But to stick to your question:
[quote author="Gerolf" date="1307947031"]why should it offizially be supported by Qt now?[/quote]
As you might have heard, Qt is used in the development of commercial applications. Producers of these applications will like to see their applications work on Windows 8 as soon as Windows 8 gets released. However, before a commercial application can be released it needs to be tested. Although the Windows 8 builds that are currently available are not exactly the same as the RTM version will be, they already give a good indication of what will not work. As it looks currently many Qt based applications will completely break on Windows 8. Getting Qt up to par with the currently available versions of Windows 8 would reduce the number of issues when the Windows 8 RTM gets released.
The alternative to expecting some preliminary/beta support for Windows 8, would be to wait until Windows 8 is released. If the implementation of Windows 8 support is not initiated before Windows 8 is released, it will be very bad news. First we would have to wait until Qt is supporting Windows 8, and then we need to start using that version of Qt, and test our application. This would mean it will be months after the release of Windows 8, before we can finally support Windows 8 in our application. In particular if this means it would be mandatory to use Qt 5, which might not be compatible and thus may require additional refactoring of the application.
And therefore, I would like to know which version of Qt will support Windows 8, and to what extend Windows 8 will be supported in that version.
P.S. I understand it is unclear when Windows 8 will be RTM, and that there are rumors about 2012. However, be aware that 2012 is only half a year (i.e. one minor Qt release) from now. And it's just a rumor, it might be considered ready earlier, though I also expect it to be later.