If you want to play, you'll have to buy the game twice
Probably we'll have to use Japanese account in order to connect to some dedicated server in Japan or something like that. Notice online racing will become available on Christmas.
I'm fairly certain it won't matter what account you use to play online, as the PSN checks your DNS/IP address to determine what servers you have access to.
For instance, I have several different European PSN accounts, but no matter which I use when playing MotorStorm, it only gives me access to the servers in the US, so I can't play against my fellow GTP members in the UK and such. The same goes for many games, but some support global online play.
So I'm fairly certain it wont have anything to do with what PSN account you play under, but what region your DNS/IP is located in and what kind of international server support there is for a game.
Seeing as GT5

is currently only officially available in Japan, I seriously doubt anyone outside Japan will have access to the online servers, but we wont know for sure until we get the game or hear from those that do.
My suspicion that there wont be any access from the US to the online servers for GT5

is what is keeping me from downloading the Japanese edition... although in fairness, I managed to have a great time playing previous GT games without ever racing online... so who knows.
So here's a question then, as I'm seeing so many people order this online, can you not just download it then if you have a japanese account? Do they not accept our credit cards or something? (I'm in Europe)
Otherwise I'm going to have to go on Renchi and order it as well.
But I'd prefer to download it when someone has made a translation and how to on how to get through the order processing in japanese. That would be way quicker for me then to have to wait for the game to be shipped here.
There have been some posts in this thread on ways of adding funds to your Japanese PSN account in order to buy and download games, and there is the following thread as well which has additional info and links: