Plain Qt 4.6 applications will run in Maemo 6.

The Maemo 6 UI Framework will offer additional / optimized features for projects interested in Maemo specific integration. Such optimizations are being proposed to the Qt team to be integrated in future Qt releases, either in the "generic" Qt or the Qt Mobility libraries.

The Maemo team (or the Symbian team) can have the "luxury" of productizing such additions / optimizations faster since the Qt team is developing a common toolkit for several platforms, and this requires a lot of extra work on development (sometimes rewritting entirely a library) and testing.

There will be enough time of Harmattan pre-releases for developers to figure out how well their plain Qt projects run and feel in Maemo 6, how interesting are the Maemo optimizations offered, how much extra effort is needed and if it's worth... And also plenty of time for providing feeedback to the Maemo and Qt teams, file bugs, etc.