• Click left and right sticks: Crouch and zoom -- just like H2, although there is a "toggle crouch" option, so that you can stay crouched until you click the stick again.
  
• B: Melee attack. Note: B may also be used to activate a subtle, but nifty context-sensitive feature. [Adam: Cover mechanic? OR maybe pick up, a la Crackdown.]
  
• X: Activate mysterious new feature which on the cool scale falls somewhere between "crouch" and "activate Fantanas on speed dial." [Adam: Again, cover mechanic, or something similar?]
  
• LB: Cycle grenade type (you have at least three types of grenade now, so they must be cycled rather than switched).
  
• RB: Pick up weapon / activate stuff / reload / use turrets and so on. [Adam: Replaces old X.]
  Note: If you hold LB while standing over a dual wieldable weapon, you'll pick it up in your left hand -- a huge improvement over the "Y" button in Halo 2. This doesn't affect grenade cycling, because you can still tap LB to cycle grenades, but you can't toss 'em while dual wielding anyway. So it's moot. After about ten minutes of play, it feels incredibly natural, with the lone downside that you have trouble adjusting when you go back to Halo 2. But more vitally, LB reloads the weapon in your left hand, individually when you're dual-wielding. [Adam: Independent R and L reloads? Nice!]
  
• BACK: Calls up the Multiplayer "player menu" -- where you can mute jerks, see gamer data and so on.
  
• START: Pause menu (which is translucent so you can adjust controls without losing track of what's on screen) and one or two other significant goodies we haven't revealed yet. From here you can also end or quit a game, naturally. [Adam: Like drop-in / drop-out co-op? Please?]
  
• D-PAD: ... currently used to activate team chat, but that's very provisional. This is being given a lot of care and attention and is tied to other matters, such as your default settings and how team chat and proximity voice work.

For the full list of tentative Halo 3 controls, check out the latest "Weekly Update".

UPDATED: Looks like there's no cover mechanic, based on this exchange between Game Informer Magazine and Bungie Studios (requires Unlimited Access):