According to http://doc.trolltech.com/main-snapshot/symbian-with-qt-introduction.html there is one command that should generate unsigned SIS files, namely "make unsingned_sis"
This command is not recognized by Qt 4.6.3. Is this new in 4.7? Old and removed?

We need a way to obtain unsigned files, preferably through Qt Creator [custom] settings. This should be done as part of QTBUG-13781 but until then, we need to know what the real status is here and look for a better solution.

Ovi Signing seems to expect Smart Insaller packaged applications to be unsigned when submitted to them for certification, and currently the only way to do that is by using a command prompt and the Symbian specific tools (signsis -u). Not user friendly and not documented.