The summit will address the issue you have asked (solely the Qt part, I am not sure about Python). Agenda is here.

Currently there is Qt4.5.3 port on Fremantle. Note that in Qt4.6 there is Qt Mobility API. That the difference I noticed between Qt4.5 and Qt4.6.

I don't know much detail, however. But according to the Qt tradition, it has been kept good compatibility for a long time.

Edit:
You probably could take a look PySide, they already support N900:
http://www.pyside.org/downloads/