Are you interested in Far Cry 3 and/or Assassin's Creed 3? Are you a PC Gamer that uses Steam as his primary shop? Do you live in the UK? If you answered yes to each of these questions, you are boned. Or, to put it more eloquently, you can't buy these games through Steam, and nobody will say exactly why, though there's plenty of speculation.
Ubisoft has stated it's "been in discussions with Valve" about the titles and that "for the time being" this the situation.
It's possible the issue is related to a trend that'd been going all earlier this year, in which all kinds of games were being witheld from the Steam UK shop due to a court ruling relating to used games (which Valve opposes, not wanting used digital games to become a thing). It's also speculated this could be another case of retailers putting pressure on publishers to prevent them from putting games on Steam, either temporarily or permanently.