IT Cutting Edge now it is CLOUD.
For big and transnational mobile operators - cloud (distributed) architecture for mobile network, global roaming, picocell, femtocell, Microcell, Wireless access point already exist.
And of course, for this Cloud architecture we require Cloud Billing :-).
Mobile operators in some countries use common core network - it's cheaper for each operator. Cloud technology is cheaper too. Many ways to implement this in mobile telecom.
But, for the first, we need to solve some access and security problems regarding this direction.
For further reading see my site - CloudsTele.com - information about all types of Clouds for Telecom industry.