Could this indeed be a killer app? It could be. As a small business my mobile phone is my lifeline. Indeed at Techaisle we all rely on our mobile phone. for inter-office calls and for communicating with our remote teams, we use Skype. So the combination of the two would help tremendously.
It is intriguing to think about what this does to a unified communications strategy of companies like Microsoft and Cisco. There are some differences and a comprehensive UC solution offers much more. But the low end of the market - very small businesses may find the above solution just good enough.