WhatsApp is rumoured to be working on native Windows and Mac apps. According to the document obtained by a Twitter account, WABetaInfo which posts information on upcoming WhatsApp builds and releases. In this case, the screenshots, which come from an unidentified source. seem to show download buttons in the works for Windows and Mac versions of WhatsApp.
While we have not confirmed the above images as coming from WhatsApp, There is possibility that WhatsApp would be developing the service for Windows and Mac desktop users as they have been very open on releasing apps for a variety of platforms. Hopefully we will have more tangible information on this as soon as possible.