Actually I wouldn't be surprised if Nokia decided to test WP7 on their already existing test hardware, the N900, before they start to make new phones. On the other hand, a Qt GUI imitating the Metro interface would be easier to write than port that crappy OS to the N900. Also based on the WP7 system requirements the N900 doesn't have a fast enough CPU (600MHz vs 1GHz), enough memory (256MB vs 512MB), lacks hw buttons (qwerty does not count in this case), and as far as I know WP7 currently runs only on one specific hardware, thus the strict requirements.