Once you unlock dk's on any of ur chars, it will be available for your ACCOUNT. meaning all races, factions, realms, servers. also ANY class can become dk. ANY. Oh ye and its true what one guy said, your neither horde, nor alliance, no matter what race you make. your going to be neutral and be able to talk to both factions, but not be able to go to major cities of ally and horde eg. Orgrimmar, Stormwind, Exodar, Undercity etc. Your major city will be the major city of Northerend, which i forgot the name of. And dk's start at level 52, which is the exact level the death knight quest becomes available to you.
Heres some other info about dk's:

First part of dk quest: Escape from Stratholme!: finish straholme in 45 min or less =P

Last part: kill a blue dragon and rez him with an orb then kill him in skeleton form (i dont know name).

Classes dk's are alike to: frost mages, warriors, warlockz, and shadow priests.

oh ye and i just want to add it says on wotlk forum that when they were talking about pandorin and naga, they didn't mean they will be playable in wotlk. they WONT be on wotlk so tell your town not to start a big rampage and kill blizzard ppl xD and if you dont believe me look at the trailer. it says new proffesion, continent, class, new dances, new hairstyles, new emoticons, siege weapons, destructable buildings, but it doesn't say new races