The Playstation Network was down during April -Jun 2011 and is now back up worldwide
It is possible to do, but would need a seperate memory card from the one used to access the US PSN store.
No, a Japanese PSN account can only be funded with a Japanese PSN card, one with a yen amount on it.
It's actually known as a PSN ID. http://www.us.playstation.com/PSN/mediago
WHAT DOES PSN STAND FOR OR MEAN
by PSN you need a account to PSN
The PSP-3000 can access the PSN, yes.
You can't. PSN stands for PlayStation Network. You can't connect a DS to PSN.
No you can't, PSN cards are country and currency locked. You cannot redeem German PSN topup cards on a UK PSN account.
No, PSN is free and there is no indication that is going to change.
Yes, there is a PSN link on the Vita's LiveArea.
The password is whatever you set it as when you registered your PSN account.
you need to have internet to do that then you sign up for psn and your there