AC2 had DRM that originally required a constant internet connection, and later required an internet connection at every launch instead. This is a dealbreaker for me as I often game without internet access. Assassins Creed Brotherhood is currently on sale. Does it contain the same DRM?
Wie Assassin’s Creed 2 verwendet auch Brotherhood den Game-Launcher-Kopierschutz, den Ubisoft allerdings leicht entschärft hat. So müssen Sie beim Klettern und Meucheln nicht mehr online bleiben, nach der ersten Internet-Aktivierung läuft das Spiel auch im Offline-Modus.
And from Gamespot
There are a number of key differences between Assassin's Creed II and its follow-up, Assassin's Creed: Brotherhood, but if there's one that some PC enthusiasts will appreciate the most, it's that Ubisoft's stringent copy-protection scheme has been jettisoned.
And from StrategyInformer
Fortunately, Ubisoft has seen sense this time around, and although certain security elements are present, it's barely noticeable anymore and definitely doesn't feel as intrusive. The game requires a one-time activation to begin with, but after that you can play the single player offline whenever you desire. The multiplayer requires you have a connection (duuh) and some of the DLC does too, but that's it.
This seems to indicate that there is still an online activation needed, but that afterwards no online connection is needed anymore. So you don't seem to need a permanent internet connection to be able to play the game.