Ubisoft says there'll be significantly more unlockable goodies in Assassin's Creed Brotherhood than before.

This bonus content, which includes stuff like Florentine Noble Attire, Altair's Robes and the Hellequin Multiplayer Character to name just a few, is unlocked with Uplay points - currency earned by playing Ubisoft games (not just ACB).
Ubi says there are more unlockables here than you can afford with all the Uplay points available in ACB, so choose carefully or you'll have to play another Uplay-enabled (Ubisoft) game to get everything.

Here's a trailer showing how it all works, as well as giving you the first look of the Hellequin multiplayer character.

[Source: CVG]