My wife tried to register me on mixi.jp, which is the Japanese equivalent of everyone’s secret vice and favorite soap opera, facebook. Mixi has been an invitation-only affair until recently, but will soon open up to anyone (just as facebook did a couple years back). I tried to register, but couldn’t get past the first page. Why? No cell phone in Japan. The mixi registration page asks for (demands, actually) a cell-phone mail address (i.e. 0123456789@docomo.co.jp). No keitai, no go. Frustrating for us no longer in the Empire.
It soon dawned on me, however, that this may be a very convenient way to handle security against spammers, robots, duplicates, and oher malcontents: all keitai mail addresses are unique. To get another address would require one to get another cell phone– not a cheap proposition. As such, every person can really only register on Mixi once (twice if they have two cell phones, like many Japanese business people do these days). Moreover, Mixi now has a new channel to keep everyone together and in touch with their Harajuku comrades before they all meet-up down on Cat Street for sushi and costumed be-bop dancing.
Genius. Frustrating, but genius.
Can I borrow anyone’s cell-phone number for registration?
I promise I'm relevant 
[...] always accepted and centered on people making aliases, which seems like a protection of privacy (a big point for Japanese), but ultimately prevented [...]