This forum thread talks about a person who dinged 70 while still back in Nagrand, and while I didn't finish off the run to 70 that early (he probably did lots and lots of instances), it is possible to bend the leveling curve a little bit, especially in Outland, where there are so many quests to go around.
So how far have you bent the leveling curve? Early on, it's pretty clear where to go to level up -- there are only a few areas you can go into at each level, and while there are definitely more quests than you need to do (especially in the newer Dustwallow Marsh content), things are pretty laid out for you. In Outland, though, things get a little squished -- Blizzard really went overboard with quests, and so it's possible now to hit the last level without ever seeing one or two of the zones.
Fortunately, any XP that you would have gotten at 70 is translated back into gold on a quest reward, so even if you've finished early, there's still lots of reason to go back and see what's out there (and there is some must-see stuff later in the game). But how off has your leveling been? Anyone hit 70 even before Nagrand?