Finally hehe
here's the source code for whatsapp built in java :
http://www.4shared.com/rar/C3VN9BnV/whatsapp.html
(along with some other files)
things needed to build whatsapp :
- getting the username
- knowing how the password is generated (i think i figured that out in the Utility class something about reversing the imie)
- the registration request (request for sms depending on phone# for user/password)
- the verification request (what is actually sent back after the verification)

extra : file transfers and groups