I'd strongly caution against adopting Viber, it's even worse than the Skype on N900 - I do not mean the technical implementation, which is quite a feat, but the backend of the closed source application (where the Viber is the same story here). I believe the Yappari 1.0.37 on N900 is amazing (Thank You Scorpius & Tarek);
An alternative would be just to have a WhatsApp "plugin" for Pidgin Messenger/Purple (that's been around for quite some time) including the OTR feature.
Maemo 5 - these days is the "Unicorn" platform enjoying quite unique level of openness and transparency and bringing more "closed" stuff is not the way to go.