I'm sure this new subscription implementation will be closely watched by others. Obviously, its competitors could be looking at how well Whatsapp does with this and if users are receptive. It's got a big enough market share that it could stand to lose a few that might jump ship.
However, social services that offer messaging or even updates like Path could look at this and see it as a potential source of revenue in the future. Yes, even now with Facebook charging users to broadcast their updates, they may even get in on the game. With hundreds of active users, Facebook may be looking at millions in revenue.
Of course, it could all back fire on Whatsapp as well. I'm hoping that they'll find success with this model. I'll explain why at a later time.