It would certainly be nice to have WhatsApp available as a plugin for Maemo. But it is 3rd party software, and not free for all platforms (for Symbian they charge 2- 3 dollars per year after trial period), I seriously doubt they will grant anyone on these forums acces to the software code. And I may not advise you to hack this app in order to get the code and port it, that would be stealing.

Or am I wrong?