Can the Hulk be suffocated or is he just completely invincible?
Yeah he needs air all right, the original Hulk was a bit of a mindless brute who tossed around tanks and survived gunshots
Then his strength got pumped up like crazy and he lifted a mountain in a story arc called "Secret Wars"
but he still needs air
Finally in recent years Reed, the Sorcerer and Tony Stark got stick and tired of this idiot turning into a giant muscular green monster so they strap him on a rocket and fire him into space. ..I think Xavier and Blackbolt were involved in a lesser extent but anyway they blast the green nuisance off to the stars. What planet? That's not important. The important thing for Reed et al is Hulk was out of the way
Anyway he ends up on this planet where he experiences a whole world of pain, has to do Russell Crowe impressions fighting to death in arenas.
http://www.youtube.com/watch?v=cf13JRZKspM
[spoilers]
He gets a gal preggers, fights more wars, they think he's a savior but no, and finally sees his new planet get nuked
[/spoilers]
...Finally he's thinking about getting off the planet
By this stage Hulk's muscles are popping out his ears and he's on full on roid rage. Now he's so strong he's superman strong, he doesn't need to breathe in space anymore.
But he never figured out the flying thing.
Don't worry the Green man tapes himself onto the side of a spacecraft and points it back at Earth.
Revenge time
http://www.youtube.com/watch?v=ItVMaE6x6hM