Jan Chipchase works in the User Experience Group at Nokia and on his blog (some time ago) asked the simple question and spent time studying 17 urban dwellers in San Francisco, Berlin and Shanghai and Tokyo with shadowing, home-interviews, plus 129 street interviews and numerous observation sessions. His research, written up as an article on his blog (along with his colleagues) came up with three behavioural insights such as the Center of Gravity, Point of Reflection and the Range of Distribution. A PDF presentation he gave at DUX 2005 is available from his site - ‘Mobile Essentials - Field Study and Concepting’ (download paper, 0.4mb).