And that's interesting, because it means that, at least right now, Blizzard is committed to implementing Death Knights throughout the entire game. Though they're only a Hero Class, Death Knights are the first "new class" that WoW has ever seen, and so we'll definitely see some interesting changes throughout Azeroth. Every class has their moment to shine in each instance, and so if Blizzard wants to give Death Knights their chance to step up in even the "old" instances like Blackwing Lair, lots of changes are probably coming.
Of course, one factor of this, however, is that Death Knights won't start at level one-- they'll be starting at a higher level when first rolled. From what we know, Blizzard hasn't actually chosen that level yet (remember as well that when Death Knights appear ingame, the highest level will be 80), but if they're planning on changing around a level 60 instance to include Death Knights, odds are that you'll have at least twenty levels to go on your Death Knight before you can reach level 80.