I also thought that AC was not down on that play, but in rewatching the game today I noticed a replay of the catch at the beginning of the second half. That replay was from a different angle than what was shown in the 1Q and if you go frame-by-frame you can see AC's left knee hit the grass as he is rolling off the DB. Additionally, the ref is in a good position to see the knee hit the ground so I believe the right call was made.
What is even more interesting is that it appears that Vitale's right foot goes out-of-bounds at the 4 yard line on his TD reception and the play was never even reviewed. Nice to get a break for once!
Go Cats!!!