The Candle herself Nethaera has recently posted up a F.A.Q. on a lot of questions that you might expect people to ask about the upcoming new class.
Posted by Nethaera (Source )
Q . What races can be a death knight?
A. Every race has the availability of being a death knight.
Q. Who is eligible to create a death knight?
A. Players who have at least one level 55 character on a realm may create a death knight on that realm.
Q. What if I have a level 55 character on one realm but want to create a death knight on another realm to play with friends?
A. At the launch of Wrath of the Lich King, you will only be able to create a death knight on the same realm where you have an existing level 55 or higher character. In the future, we plan to allow the creation of a death knight on any realm (provided you have at least one level 55+ character).
Q. Can I create a death knight on one realm and transfer it to another realm?
A. Yes, you will be able to transfer a death knight, using either a free character move or a paid character transfer. Normal fees still apply.
Q. Can I create a death knight on one account and transfer it to a new account?
A. Death knights can be transferred with the same fees and restrictions as any other character.
Q. If I move a death knight to another realm now, can I get a refund later when the option to create a death knight anywhere is available?
A. We will not be giving refunds to players who have relocated a death knight to another realm prior to the open creation point for all realms.
Q. Can I create a death knight on the opposite faction?
A. You will be able to create a death knight on the opposite faction provided that it is on a PvE realm. The standard restrictions for character creation apply for PvP realms.
Q. Is there a limit on how many death knights I can create?
A. You will only be able to have one death knight per realm at any one time. Should you delete a death knight, you will still be able to create a new one.