It may be a combination of Vampires and Imaginary Friends, with you needing to be bit to start the transformation, but after that you can transform at will as Imaginary Friends do. Or it's possible that you can only change at night or at certain phases of the moon.
Assuming that any of this is actually going to happen, of course.
My guess is we'll get nightly transformations, like in the sims 2. The moon would have to cycle through phases incredibly quickly for werewolves to get a decent amount of transformations on the more standard settings for life-length.
The real question though is how lycanthropy will transmit itself through the population. In the sims 2, you had to first get infected from the leader of the pack, and then choose to have werewolf sims savage other sims in order to transmit the infection. (which fortunately, did not require particular relationship scores) Given that pets (which the leader of the pack technically was) has already been added via expansion to the sims 3, I'd say it's unlikely that the leader of the pack would return as the initiator of lycanthropy. Chances are, we'll have to get infected by townies this time around. My guess werewolves will, like vampires, have a means of turning people requiring high relationship values. Hopefully though, they'll keep the savage interaction available to at the very least player controlled werewolves.