1) libqt4-core seems not to be in any repository, I can't "apt-get install libqt4-core" and "apt-get install libqt4-gui". The latter complains about missing dependency "libqt4-core". I think it was pre-installed on the N900. Where do I get it now?
2) How can I switch the Qt version when running an application? In scratchbox I can compile against 4.5 and 4.6, but when I tried to run the 4.6 application on the N900 it always told me that I can't mix different Qt version. That was why I removed the 4.5 packages, and then the application worked. Is it possible to have 4.5 and 4.6 installed on the device and choose one when running an application?
Back to the original topic, I recommend you top ask in the official support channel: http://qt.nokia.com/developer/qt-for-maemo-developers ("How to provide feedback"). Not only because there you ave the Qt 4.6 for maemo experts, but also because this way they realize easily the libraries are used by more developers, they put more attention to fixes and new releases, etc.