The Pokémon Global Link website has announced that the site will be unavailable from September 25th, 2012 until October 3rd, 2012. The update should be similar to the one Japan received when Black 2 and White 2 were released there in the summer. Below are the full details.

Black 2 White 2 Update News

The Pokémon Global Link is getting a tune-up in readiness for the Pokémon Black Version 2 and Pokémon White Version 2 games.

Starting September 25, the Pokémon Global Link site will enter an extended maintenance period. During this time, none of the services of the Pokémon Global Link will be available, including the Pokémon Dream World, Global Battle Union Wi-Fi Competitions, and Game Sync linking. Remember to wake up any sleeping Pokémon before the maintenance period begins!

The Pokémon Global Link will be available again starting October 3, in time for the Pokémon Black Version 2 and Pokémon White Version 2 games, which launch on October 7!

Source: http://en.pokemon-gl.com/#/information/441

I really wonder why they need such long maintenance periods. I can't say I know squat about that sort of things, but I'm surprised by how long they are... Do they have some big-*** monsta databases or something?