iOS WhatsApp users will be transitioning to the financial model that the app already uses on Blackberry, Android, and Windows Phones. Previously, iPhone users needed only to pay the one-time upfront fee of $0.99 for the app and could use it thereafter free of charge. However, WhatsApp's announced that it will now be available to download for free via the App Store. WhatsApp iOS users will now pay $0.99 per year of use, much like users on all other mobile platforms.
However, nothing is going to change for current iPhone users. Those who have already purchased the app will remain on the current financial model and will not be charged the $0.99 per year fee for the app.