Viber has been changing their minds on a regular basis about finally supporting BlackBerry 10. First they planned on releasing a native app and then the reneged on that promise. After that they decided to possibly bring the service to BlackBerry 10 and now it is finally here. The downside is that instead of a native app the Viber app for BlackBerry 10 is an Android port. On one hand this is good because it will have feature parity with their mature Android app but on the other hand it will be nowhere as good as a native app.
Either way you can now get Viber in BlackBerry World at this link. We held off posting it because it seems like it is not available to all just yet.