Just a minor clarification on terms:
In ANCS the iOS device is the server and another device is the client. A server owns the information that the client wants to interact with, like text message notifications. Since the RFduino's current service also acts as a server, not a client, it isn't able to act as a client to ANCS.
The terms Central/Master designates the device that controls the timing of the radio communication whereas Peripheral/Slave refers to the other device. Initially the Central does the listening and the peripheral does the advertising. Once they've connected they are in a Master/Slave relationship. I believe that in ANCS the iOS device is still the Master (controlling radio timing), but it "serves" the data to the Peripheral, which acts as the client. Note that "Central" and "Peripheral" refer to roles that devices can have. The terms Central and Peripheral are used when devices will establish a connection. There are also roles called Broadcaster and Observer.
I don't mean to be pedantic, but BLE is littered with terms with subtle differences. It can certainly be confusing.