Seems like, as of some update after the original report above, Surround stopped working for this game. It still allows me to select my usual Surround resolutions, both in the config and in-game, but the actual game displays only in the middle monitor. The two side monitors go to sleep. I don't have this problem with any other game. However, I only just now bought this game, so I never tried it before Surround support broke.
While googling, I found another Ubisoft game in which they broke Surround with a subsequent update:
http://forums.ubi.com/showthread.php/15 ... ate-Forums