@Ksemanr: It doesn't seem. It is northern. It's directly confirmed IN-GAME (not implied, but confirmed) that Sinnoh lies to the north: when you play Sinnoh Sound on your Poké Gear radio, DJ Ben refers to Sinnoh as a northern region.
Pokémon has never been a launch title as of today. But who knows, things might change.
I meant that it always seemed to be northern, even before they confirmed it in HG/SS. Sorry for the confusion. I should've worded it better.