someone on another forum mentioned that the assembly that the pedal attaches to is plastic & there is some wiggle room in it. they resolved by putting a wedge or something to take up the movement
eta:
http://www.gm-trucks.com/forums/topic/167606-gas-pedal-flex-solution/
That's not it. The Denali does start up right away, but then there is a delay. I heard they did this because its a heavy machine and they don't want soccer moms to kill the transmission by doing burnouts at stop lights.