"Charles," who helped with programming on the game, then commented on that post with a code to unlock Boss Rush mode! "I think the code for Boss Rush was something like right, right, B, R1, right, right, B, R1 ... or something like that." We can confirm that the code works in the XBLA version of the game. Total Video Games reports that it works on PS3 if circle is substituted for B, and also notes that Ubisoft was set to release this code as part of a "newsletter incentive." A code to unlock a zombie survival mode was previously released in a trailer.
Later in the comments, Boutin says that "There will be a downloadable pack with a new character and maybe the extra game modes if they aren't unlockable right now, but i'm pretty sure that will be it."