Both the application and its protocol are closed, so WhatsApp, Inc. are the only ones who can port the application. You'd likely be better off contacting the company directly rather than posting here if you want their attention.

WhatsApp has made a beta of their application available for S60 3rd & 5th editions as well as Symbian^3. If the company hooks a good number of Symbian users into their $1.99/yr service plan, and if MeeGo sells reasonably well, then a Symbian and MeeGo Qt version of the app doesn't seem out of the question.

Should that come to pass, it will still remain to be seen if that helps vanilla MeeGo on N900 or MeeGo-Harmattan on N900 users. It will be interesting to see how the DRM support available in future MeeGo handsets, but not available in the N900, plays out in the commercial application marketplace.

The other possibility is that someone might reverse engineer the WhatsApp protocol and develop a compatible plugin for Pidgin. Most likely that would result in a game of "chase the changing protocol" though, as proprietary IM service providers have a history of disliking people crashing their private parties.