Example:
1) create "Mobile Qt Application" project
2) build for Symbian Debug
3) build for Simulator Debug
4) add a pushbutton to the form
5) build again for Simulator Debug
6) run -> no pushbutton

This happens because the Simulator build uses ui_mainwindow.h from the project directory and not from the shadow build directory. I tried to workaround this by switching Symbian build to use shadow build as well, but that doesn't work as Qt Creator always switches that option back to no shadow build. So the only possible workflow seems to be: whenever a Symbian build is made, push the sis file to a phone and then make clean immediately, and only then build for Simulator again.