not quite, From what I can tell, you'll work to 80, do the quest, and then when you make a new character you'll have the option of picking death knight, who will START between 55-70
So its a completely different class? It would seem a little unusual to have someone invest the time and effort into getting their main to 80 only to force them to start a brand new guy and dump them into level 55-70 with no gear, money or mount.
I was under the impression becoming a Death Knight was really just a fancy way of bestowing one (a few?) Epic power(s) upon your guy that would be tailored to synergize with your current class.
I like my theory better then yours.