According to numerous reports and several personal experiences, much of the Burning Crusade content is offline on several servers, both in the U.S. and the E.U.. When attempting to access a character in any of these locations you will be presented with an error that says "World Server Down."
This is no doubt from the mass of people who have flocked to the Isle of Quel'Danas in order to see the new content. While the realms and content are being bumpy tonight, checkout our extensive collection of patch 2.4 information.
I'll update this post as the night goes on with the latest realm announcements. Stay tuned!
Update: Realms are all online, although a tad bumpy.