Traveling in Japan one gets an immediate impression of the impact that mobile devices is having here. One would be hard pushed on a train to not see someone doing something online. In addition, a number of developments (initially centered on continuing automation of ticketing on the rail networks) have led to deeper integration of pay-by-cell phone mechanisms.
It is tempting to look at Japan (and Tokyo in particular) as a vision of the future for other countries or cities. For example, cell phones in Japan are getting larger (while the rest of the world is still in, or recovering from a smaller is better approach) - this is a trend we can expect to see in the US soon (thanks in part to the iPhone). However, there are more fundamental aspects of a society that cannot be disentangled from their attitudes and integration of technology. Two come to mind in the context of mobile devices in Tokyo.
Firstly, Tokyo is all about public transport. Trains run at minute intervals and are packed. Public transport is prime info-snacking space.
Secondly, population density in Tokyo results, obviously, in small accommodations. One consequence of this is that people, and young people in particular, spend a lot of time outside their place of residence, thus providing many user needs for mobile devices and information services.
The point I want to make is that these two factors, I believe, lead to increased use of mobile devices and, importantly, they won't be replicated in most US cities.