WPCentral managed to rip the XAP for the WhatsApp beta in Marketplace and have published this video review.
They note the app is fast, but seems to be rather stripped down. The app does however appear to support most of the features of the app, including group messaging, notifications and attachments.
WhatsApp is expected to hit marketplace very soon, but the curious can find the XAP at WPSauce here (dev-unlocked device needed).