Looking at applications today, it is obvious they are EITHER implemented in the network OR implemented on devices (thinking about iphone apps etc.). I can imagine applications which are a joint implementation in both domains, i.e. a combination of device applications and network applications. How do the service brokers help in developing/implementing these types of applications?

This is a great point, and one where service brokers may change the typical approach.  A service broker exposes network capabilities, such as presence and location, or network services like find me/follow me, to the ecosystem of developers in the device domain.  The sizzle today is on apps though, which the subscriber interacts via a smartphone for example.  By combining the network capabilities easily exposed via Web 2.0 APIs to developers, much richer offerings can be delivered to subscribers and from a service provider point of view, more of the existing network can be leverage, and utilized, thus driving affinity towards the network.

From my perspective, the goal is to drive utilization of the network by subscribers and ensure there’s value perceived and used, which then drives loyalty to a service provider.