And - this is NOT subaccount - it's still the same (main or sub) account just the Screen name could be changed. The inital usage at Japan was to allow users to enter the user name in Kanji or Kana and then have an "English" Screen name as well.
Nope - how it works (at least in Japan for now) is that you have ONE fixed PSN ID which can not be changed under any circumstances and one "Screen Name" which you can choose as you're pleased. However, all the account transactions / operation (like messages, frend or block lists etc.) are still handled via original PSN ID and even at Home for example where you'd see the Screen name above person's avatar all you need to do is to check his / her account details and you'll always see the original PSN ID as well so even if someone will change the Screen ID you can still recognize them anyway