The developers of whatsapp messenger announced a while ago that they will not be porting the whatsapp application to the Maemo5 and Harmattan platforms. Users on these platforms were a little upset until some people posted bits of information about how the protocol works (it's proprietary, not publicly documented) and after much discussion at TMO. Tgalal said he would develop a whatsapp compatible client for both the N900 (Maemo5) and N9 (Harmattan) platforms.
After a false start at TMO, Tgalal started releasing an N9 client and left us N900 users without a whatsapp client (to be fair, he wanted to work on the N9 first and when it was stable, port that over to the N900). Also, users in TMO got very impatient and somebody released a Java based whatsapp client for N900 users to use while Tgalal worked on the N9 client.
With the N900 users struggling to use the "very early build" of the Java based client, Scorpius announces that he is building an N900 native client. Released on 31-May-2012 and after multiple revision and updates, the client is pretty stable and basically works. You can participate in this "early alpha" build by visiting the Yappari thread in TMO.
Ohh! For N9 users, you can visit the wazzap link for the installer.