The wiki contains instructions:
http://wiki.maemo.org/Packaging_a_Qt_application

To build the armel target you need to add the Nokia QT SDK's integrated Madde install to your path and run 'mad dpkg-buildpackage'

Note that you only have to follow all the steps the first time to 'Debianise' your app. Once done, for subsequent releases just modify the debian/changelog and run 'mad dpkg-buildpackage' again.