Horde or Alliance or Neutral?
Death Knights will be available to both Alliance and Horde players. What races can be Death Knights has not been announced yet, my own speculation is that any race will have this choice.
For more information on Death Knights and other announced features for Wrath of the Lich King, check out this link:
http://www.worldofwarcraft.com/wrath/index.xml

December 12th, 2007 at 3:59 pm
As far as i know the Death Knight will be available to players who reach a certain level(im assuming lvl 80 but not sure) and both factions(horde and alliance) will be able to play the death knight.
References :
worldofwarcraft.com
December 12th, 2007 at 4:04 pm
Deathknights will be available to both horde and alliance. The current plan (released information by blizzard is always subject to change) is that any player can unlock the deathknight class by performing some lvl 60ish quest with any one of their current characters.
Blizzard has said they did not want players to rush through brand new content (lvl 70-80) when the expansion is released, just to unlock the deathknight. They want players to take their time and enjoy the new content, so the deathknight class can be unlocked before playing past lvl 70.
References :
December 13th, 2007 at 8:45 am
Death Knights will be available to both Alliance and Horde players. What races can be Death Knights has not been announced yet, my own speculation is that any race will have this choice.
For more information on Death Knights and other announced features for Wrath of the Lich King, check out this link:
http://www.worldofwarcraft.com/wrath/index.xml
References :