While traveling during his ultimate, is Nocturne targetable by friends or foes?
Nocturne is immune to Crowd Control effects when he jumps. When the enemy tries to CC Nocturne in-flight (even without targeting him, such as using Janna's Howling Gale), the message "Cannot Be Disabled" will appear above him.
Shen can teleport to an in-flight Nocturne because Shen's shield and ult are not CC.
EDIT: Michel found this great Youtube clip that shows this in action.
Conceivably, Nocturne could be targeted by an auto-attack in-flight. But since the enemy team usually doesn't have any vision and he moves quickly out of auto-attack range even if there is vision, this rarely happens.
If you watch Nocturne's health in the Youtube clip carefully, you'll notice that even though he gets hit by Amumu's and Galio's ults, he doesn't take any damage. This suggests (doesn't prove) that he is immune to damage during his jump. It'd be great if somebody could test this and post the results.