Hi,
We have briefly tested an ArcGIS Runtime application on the recently released Windows 8 early beta and will be performing additional testing over the coming months. However, Windows 8 is not scheduled to be released until after ArcGIS 10.1 (including the ArcGIS Runtime) and therefore it's not possible to say at this this time what the status will regarding certification. The good news is that we're planning to enter an "off-cycle" release schedule with the ArcGIS Runtime after 10.1 which means we'll hopefully be able support Windows 8 soon after its release.
Cheers
Mike