Maemo/Meego community has been working to fulfil the wish of many N9 users to have 3rd party application Whatsapp on their phones. The Meego version of this application has not been released by the parent company even after many petitions and requests from N9 users on many forums.
But now thanks to the vibrant Maemo community, ported Whatsapp application for your N9 and N900 devices will soon see light of the day. The developement is rapid and thanks to some posts in Maemo forum, it seems beta can be released next week.
Below is a snapshot of the ported application on N9, which is considered to be in alpha stage now