Was really curious.

I've got a guild member who quit the game before Wrath.
When he quit, he decided to delete every toon on the server.
He has no toons on any other server.

Long story short, he came back and choose the same server.
He installs Wrath and is able to make a Death Knight at 55th, with no other toons on any server.

I guess having had a few 70's in Burning Crusade, set a flag so to speak.
So even though there were no other toons at all, at DK creation time, previously having had a 55+ let him create one.

Wasn't sure if DK's were unlocked on even new accounts or not.