Is CCleaner supported on Windows 8.1 when it's comes out?
Only Piriform can tell us that, so we'll have to wait for the announcement.
I'd imagine it should work fine once it's out - especially since the preview is available for users and developers.
Even if it isn't 'officially' supported at first, it may well work since it's only a minor iteration over Windows 8, but thing can go from good to bad pretty quickly.