Anyone who pre-orders the game at any participating retailer will receive the M55 Argus Assault Rifle (and since I've already pre-ordered at Best Buy, looks like that's what I'll be getting).
Anyone who pre-orders the game at EB Games will get the M55 Argus Assault Rifle and the N7 Warfare Gear.
Anyone pre-ordering via EA's own Origin will receive the M55 Argus Assault Rifle and the AT12 Raider Shotgun.
Full details and a brief FAQ can be found here, and links to screenshots and videos are also included. You can also check out the videos below.
I personally find the enemies revealed in the videos more interesting than the pre-order bonuses themselves; what appear to be Turian Husks and Krogan Husks.