38 days from now, Wrath of the Lich King will be released, and hundreds of thousands of people will be chomping at the bit to roll Death Knights, WoW's first new class. Blizzard knows this as well as anyone, so they've posted a FAQ on DK character creation. If you've been following the news for months, there is little new here, but it's good as a recap, because these questions are, indeed, very frequently asked. So go on and brush up if you have questions about what races can be DKs (all of them), or whether you can make it the opposite faction of your existing characters (yes, unless you're on a PvP server).
There is, though, one bit of news in this post. When Wrath launches, you will only be able to create a DK on a realm on which you have an existing character of level 55 or higher. Wryxian, that maverick crocolisk CM, promises that in the future, the option will be opened up to create a DK on any realm, provided you have at least one 55-or-higher character somewhere. The previous plan was to make this the rule at launch (55+ = DK on any realm).
The only reason I can think of that they would make this change is that they were having difficulties from a technical standpoint; maybe it's non-trivial when trying to create a character on one realm to check that account's characters on other realms. Anyway, I doubt this will affect anyone too much. I for one am certainly going to make a DK on my main realm before I branch out to realms that I don't have any high-level characters on.