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Strange, when I went to Japan (in 2003 BTW) one of the things that impressed me was that phones didn't bother with the crap that SMSs/MMSs are: they just use e-mail.

That's also transparent on Japanese phones. When you send a message to a user on another network and it can't use SMS or MMS, it sends using an email-like address but actually gets conveyed and received as MMS. No one uses regular POP email on their cell phones, at least no one I've ever met. I suspect what you thought was email was just the MMS-gateway address for senders on other networks.

It'll be a real step downward, interface-wise, if I have to constantly switch between the Mail app and the SMS app to keep track of a conversation, and if MMS messaging doesn't work or get received. Not to mention that the iPhone shows no signs of showing all the extra characters and e-moji that Japanese phones use.
 
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