Short answer: No

Middle answer: F*ck NO!

Long answer: Whatsapp encription sucks. By a wifi without password, somebody could see all you send and receive. By 3G just your phone company. If somebody get your device, he/she would be able to get all your whatsapp conversations. Privacy policy of Whatsapp could change when they want and they wrote that they won't tell about the changes and that you must read them at their web from time to time.

Spotbross or Gtalk are safer, they use good encription. There isn't Spotbross por N900 or N9, Gtalk support is integrated and could be used from any jclient wich support jabber protocol.