If you're planning to make a Death Knight on your favorite server, and you're an altoholic (I myself am guilty on both counts), you've probably already thought about your delimma, and Bornakk drove home the point today: There are still no plans to make extra character slot for WoTLK. Just like the problem you may have had deciding what characters to delete to make your Draenei or Blood Elf, you'll have to make the same decision again.
I suppose it's understandable. I'm sure it would be a rather large jump in server storage space needed to give everyone a massive amount of extra characters. Still, It's going to be tough for a lot of people to delete or transfer a character so they can play their new Death Knight with their friends, I imagine.
Bornakk does point out that at least, with the level 55 creation level, you can start the Death Knight on a seperate server and get a feel for it before deciding if you want to delete or transfer a character on your main server. Still, I know that half the fun for me is getting a good name reserved for my character, so I've personally already deleted a lowbie Draenei Paladin from my main server in order to create a placeholder character with my chosen name.
[Thanks for the forward, Kevin!]