Ok i was watching the trailer in slo-mo and when Prime gets punched in the face it looks like he has something on his back. I was wondering maybe he is combined with jetfire. Also the bot that is punching him is very old looking so i was thinking maybe this is the Bot in the Movie Poster. Also the Seal that said they where protecting optumus's body while waiting to air-lift it out maybe this is the scene when he goes down.
Refer to the "Megatron?" thread here: http://www.tfw2005.com/boards/transformers-movie-discussion/217466-megatron-2.html#post3274862 While you may be right about Prime going down, upon way too much analysis it's clearly that foot hitting him, and I stand by it being Megatron's.
