From WOW Insider:
We know that the DK will start out at a relatively high level. It is not known exactly what that level will be; probably somewhere between 50 and 70. My guess would be 55, if that turns out to be the level of the DK-unlocking quest. The stated reasons for DKs starting out high-level are that players are tired of the early levels...
Not only am I tired of early levels, I don't have an alt over level 7, I don't think. Start me at 55, though, and I just might enjoy it. That's worth a shot.
I'm looking forward to the Death Knight. I played a few Blood Elves and Draenei, but the fact that the starter missions are geared to newbies made it less than enjoyable. At 55, you're pretty good at the game already, so even if you group up a bit, that should make for a fun run.