If you haven't yet, go check out Michael Mace's European vs American mobile phone use post. He picks up on different things than I do in my forthcoming book, Designing the Mobile User Experience, but he appears spot-on (even in his disclaimers, which basically mirror mine).
Other good sources include some ethnographic research on international differences, various What Japan Thinks articles, and this Trends in Japan article and others from the site. There is a lot of good reading available.