Guys like Serebii, Pokebeach, bulbagardens know everything. They get press release kits from Nintendo, also sources from People who are direct contact with the game somehow and passing on the info to them.That's not really exciting IMO.
Now if it was a johto starter...
EDIT: How does he get the info anyway?
Serebii was given all the Pokemon black and white names and details before the game was out. He chose not to leak any out because of TOC with Nintendo. Instead some Spanish guys who worked for Nintendo magazine leaked all the black/white Pokemon.
I have no doubt these guys know a lot more right now, they just wait for the info to go public somehow before posting.