Viber added a beta version for BlackBerry and Windows phone 7 in app world and windows phone market place to download. For now users are only send text through Viber Beta app.
For full version of Viber app you have to wait that allows you to use free call function over the wifi, 3G and 2G. Viber is most favourite among mobile users and takes place at Second the first is Skype.
Here is the interface of Viber beta on windows phone and blackberry:
Download Link for Windows Phone 7
viber download for window 7 omnia call Nd text.