In Symbian, we need to install qt_installer.sis before installing qt app.
In Maemo, does it have the same problem?
Maemo is way too different from Symbian to share any problems with it. Qt is part of firmware image for N900, so you don't need to install it separately. However, at the moment available firmware has Qt 4.5.3 and upcoming release (a.k.a. PR 1.2) will have Qt 4.6.2 included.