The beta client for WhatsApp picked up an update to 2.16.318, bringing the ability to make video calls. When you tap the call icon, you now have the option of choosing between a voice call or video call. The interface itself is similar to what we've seen with the voice calls. You can start a new call from the Calls tab, or from within a conversation window by selecting the call icon.
For video calls to work, both parties need to be on the beta channel. Otherwise, you'll just get a message saying that the recipient needs to update to the latest version. The update is live for WhatsApp beta users, and should make its way to the stable release in the coming weeks.