There's a few layers here...
Square Enix Account
|-> FFXIV Service Account 1 (PC Registration Code)
|----> FFXIV Service Account 1 Characters
|-> FFXIV Service Account 2 (PS3 Registration Code)
|----> FFXIV Service Account 2 Characters
The requirement to purchase a PS3 version if you want to play your 1.0 characters on a PS3 is only to get you the software. At that point you can play use any characters on Service Account 1 either on the PC or PS3, but not at the same time. You could throw the registration code away, give it to someone else, or register another service account to your Square Enix account. Service Account 2 characters would also be able to play on the PC or PS3, again on both systems at the same time. You can however play a character from Service Account 1 on one system and a character from Service Account 2 on another system concurrently.
XI Wiki


