Update: BioWare has posted a page for ME3 pre-order bonuses on the official game site:

A newly revealed bonus is the AT-12 Raider shotgun, which will be available for EA Origin pre-orders. In addition, the Argus assault rifle has been confirmed to be a universal pre-order bonus for all retailers; even if you pre-order with Origin or Gamestop (for the N7 Warfare Gear a.k.a. "N7 Valkyrie Rifle and Defender Armor"), you'll still get the Argus.

Despite earlier indications from BioWare that pre-order bonuses for Mass Effect 3 had yet to be finalized, Best Buy and Gamestop have already posted what their bonuses will be:

Argus Rifle (Best Buy):

ME3 Argus Rifle bonus

"The high-powered Argus rifle is an excellent close-range weapon, and its bursts of fire conserve ammunition during lengthy conflicts. Others across the galaxy are adopting the Argus as their standard rifle, for both its intimidation factor and suppression power."

N7 Valkyrie Rifle and Defender Armor (Gamestop):

ME3 N7 Rifle and Defender Armor bonus

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