If you try to install Nokia QtSDK on Ubuntu using the Nokia installer (that provides a newer version than the one distributed in Ubuntu Software Center) you could get an error like this:
(Qt_SDK_Lin32_offline_v1_1_3_en.run:3126): Gtk-CRITICAL **: IA__gtk_widget_style_get: assertion `GTK_IS_WIDGET (widget)` failed
to fix it, you need to run the installer with a specific parameter:
./Qt_SDK_Lin32_offline_v1_1_4_en.run -style cleanlooks
and everything should work!