In an interview with Eurogamer, Crackdown producer Phil Wilson said that future downloadable updates may add flying vehicles to the game:

Phattso: Are there any plans to add flying vehicles via download? Or is that being saved for the sequel or, perhaps, not on the cards at all? The thought of having an agent in co-op hanging off the wing of a light aircraft I'm flying and then both of us dropping in on a boss appeals to me greatly.

Phil Wilson: Since nothing's announced for the forthcoming downloadable content I have to be careful what I say about it, but flying vehicles are something that we initially decided to steer well clear of. That was partly because we wanted verticality to be earned only through vertigo-inducing climbs and were also concerned that we might possibly break that explorative element if we threw in a helicopter or similar; but it was also because flying vehicles put new demands on a game engine that already had more than its work cut out to support the existing goals. As it happens that latter concern was irrelevant because, as you can see in the finished game, the existing engine can handle immense draw distances and rapid streaming... so airborne antics is definitely not off the table going forward.

Not exactly a confirmation, but it's neat to think that it could happen. Remember the old days of gaming, when what you bought at the store was all you were going to get until the sequel? How the times have changed.

via 1UP