GT-i9300? That's not a valid model number! At least not yet. This piece of info found its way onto Samsung's support page a few days ago, but has since disappeared.
Given how Samsung orders it model numbers, it's safe to assume the GT-i9300 will be the next phone in its Galaxy series. Here's a run down of products and their IDs, courtesy of Ausdroid:
- Galaxy S — GT-i9000
- Nexus S — GT-i9020
- Galaxy S II — GT-i9100
- Galaxy Nexus — GT-i9250
Ausdroid reckons it'll be the Galaxy S III, which sounds like a fair assumption. Slashgear predicts a Q2 2012 arrival, but that has no bearing on when we'll see it here.