In short: you can develop closed source applications with Qt for free thanks to the LGPL license. If you have something you want to release *right now* then use the Qt 4.5 port. If you are just getting started then you'd better plan for the official Qt 4.6 port that will give you the best compatibility path to Maemo 6 + other platforms.

See http://qt.nokia.com/developer/qt-for-maemo-developers for more information and support channels directly from the Qt team.