If you want to use WhatsApp on NITDroid and Yappari at the same time:

1.- Install NITDroid and Official Android Whatsapp client in it.
2.- Register your N900 phone number with Whatsapp. Complete the registration up to the end.
3.- Go back to Maemo
4.- Install Yappari if you haven't installed it already.
5.- Type exactly this in terminal (AS USER NOT AS ROOT)
Code:

mkdir -p /home/user/.config/scorpius
echo android=true >/home/user/.config/scorpius/yappari.conf

6.- Start Yappari, type your N900 phone number with country code
7.- Yappari should detect the number was already registered using an Android client and log in without any problem.