Happened to me as Amumu when taking the enemy inhib turret. Cho popped me up and I stayed there for a good 4 seconds. I actually managed to run away while still in the air somehow. Albeit, not very far. I'm assuming knock ups are on a timer so you can still move once that time is up even if you're not actually on the ground? xD
We all laughed, and the turret still went down. All that came of it was me saying in all chat "I learned to fly! <3"
I don't remember if there was a second champ on their team who had a knock up ability, but I've seen it done with only Cho's rupture, and I think that's what happened to me.