There have been several unofficial holo-themed WhatsApp Messenger apps for Android for quite some time. Now WhatsApp has officially released the app for Android with Holo UI on their website. This replaces the old Gingerbread UI. The Holo UI brings changes to the chat screen, attachment icons, expandable notifications (Android 4.1+), scrolling through contacts in alphabetical order, new settings screen and lots more. The updated app also has several bug fixes and improvement.
Since the app is still in beta, the update it not available on Google Play yet. You can download the latest version (v2.9.2294) of the WhatsApp Messenger from the WhatsApp website [APK] and sideload it on you Android smartphone.
Have you tried the new WhatsApp Messenger for Android with the new Holo UI?