There's lots of loot, and lots of it is downloadable content, inside the Mass Effect 3 "N7" collector's edition, detailed in this here trailer that also cautions supplies will be "extremely limited." Of course they will be.
But if you plunk down a preorder for the $US80 special edition, you'll get a ton of in-game extras, including skins for your squad and weapons. Shepard gets a Normandy hoodie and a robot dog when he's aboard the ship. A "classified" extra character and mission will also be available. I suppose we'll hear more about that later.
In terms of physical goods, the game supplies a hard-backed concept art book, a steel case depicting both Shep and Femshep, and a lithographic print of the Normandy.
Mass Effect 3 releases March 6.