Some of the servers are jam-packed, while others have plenty of room to grow (no labels). NCsoft has temporarily locked new character creation on the higher population servers until the lower population servers catch up. To maintain faction balance they are also keeping track of Asmodian and Elyos populations. If one side becomes too dominant, new characters of that faction are disabled until the other side can catch up.
This has led to some frustration from preorder customers who fear they may not be able to roll on a server with their guild mates. Apparently, everything will work out as the lower population servers start filling up, so our fingers are crossed. We've all seen what can happen when companies get too overzealous with their server launch plans. The situation could be worse.
Update #1 (Sep 19 @ 10:48am EDT): Looks like all servers have been unlocked, but 8/24 of them are 'Elyos Only.'
Update #2 (Sep 19 @ 12:30pm EDT): Some players are reporting they can make Asmodian characters on the 'Elyos Only' servers. This is probably due to a lag period between real-time server status and web updates.
Update #3 (Sep 19 @ 3:00pm EDT): It appears that restrictions have been lifted (see http://twitter.com/aion_ayase).