A little speculation on the controller layout for playing Halo 3. I've got a hunch that the 'special function' reserved for the X-button is to act like a 'shift' key of sorts, to be used for less common functions like Sprinting and Swapping weapons with marines.

My idea of a Halo 3 control layout would be-

Left click=crouch
Right click=either zoom or jump. I'm guessing the latter.
Left bumper=Grenade switch, reload
Right bumper=Reload
A=Action. Could also be jump like it originally was. Zoom/ change in Rate of Fire is also possible.
B=Melee(Bash)
Y=Swap
X=Alternative function(It used to be 'action when prompted' earlier but Bungie clearly implies this isn't the case.)

Press X to toggle shift, press X again to untoggle it.
Shift- A=Zoom when there's an action prompt.
Shift-Right stick= zoom/change ROF when there's an action prompt(Depends on what's being used to zoom/ROF). Could also be for peeking around corners, I guess.
Shift-Left stick=Sprint.
Shift-B, Shift-Y, Shift-bumpers= I'm guessing that shift-bumpers won't change the usual reloading and grenade swapping, but there might be something for shift-B and shift- Y. Communication with NPCs in single player, like giving some sort of command to marines? Weapons swapping? Ammo swapping? Some unlockable weapons functions?

Since the control layout has to be k.i.s.s. or Keep It Simple Stupid, I suppose the shift-B and shift-Y are going to be for rarer functions like swapping weapons with NPCs and some sort of communication with them, or calling in an airstrike like in the E3 2003 Halo 2 demo. Additional weapons functions are seriously unlikely because a highly complex weapons layout would ruin the game.