Viber SMS has now arrived and works just like regular iPhone text messaging. Messages between Viber users are free, as with voice calls. Many UK mobile users have lots or unlimited text messages included in their monthly price plan so where Viber scores is for texting international numbers. SMS to international numbers is always charged at a premium by mobile operators so a service that delivers them free of charge is great news.
Another neat trick is that although Viber isn't officially supported on the iPod Touch is works fine. You just need a mobile number on another device to register with and that number becomes your iPod Touch Viber number.
Talmon also mentioned the challenge of working in a world where mobile operators block or degrade Voice over 3G to prevent their customers from using it. In his experience some operators are determined to deny their customers the benefits of VoIP services. Whilst this may protect revenues to some extent in the short term, it will inevitably alienate high spending customers who will churn to operators that do embrace change in the market. It will be interesting to see how this situation develops and whether operators can continue to make life difficult for Voice over 3G.