Qt has been focusing on tablets through Qtopia previously, that's why no 'direct' support for Qt on tablets. Not sure about the future roadmap as Qtopia/QtEmbedded is no longer an option, but if you did your app the 'proper' way, Model-View-Controller style, the only thing you need to redo are the dialogs and almost none of the functionality itself - which *is* a major gain.
Nokia's plan is less for tablet/desktop compatibility than it is for tablet/S60 compatibility.