The think you must remember is that within an environment where there are a lot of dependencies, you must have all that dependencies mapped and all paths must be tested.
Having services, they must be used with a lot of clients, so every client should follow the test cases.
With services, you also need to pay attention to network issues. Make the traditional tests putting a lot of traffic in the network and switch off to see how it works.
Besides that, having services do not require other kind of different approach. Just control all the inputs and outputs.