To add to
fondue's remarks about iMode, firstly call charges and line rental for
mobile phones in
Japan are typically cheaper than their
landline rental & charges. This means that many people living in tiny Tokyo apartments actually use these phones as their main form of contact. In
Ireland, I have very few friends who do that - mobiles are only used to call other mobile numbers; otherwise the call becomes
expensive very quickly. Those who do use mobiles as landlines pay about double the price per month for call charges, and obviously can't stay on their phone for hours on end.
In addition to this, the fact that iMode is 'a neat gimmick' can be of value in and of itself. The social consequence of both this and the facts mentioned in the previous paragraph is that practically everyone in Japan has a mobile phone; not only do they have them, but they are constantly using them - don't they tie them around their necks? It's very difficult, if not impossible, for a culture that has grown up like this to suddenly stop using phones altogether.
Hence perhaps the one effect of iMode gimmicks in Japan is that cellphone penetration is almost complete there - the US has a very long way to go, even in just highly-populated urban areas.