No difference between an iPhone 4 with IOS5 and an iPhone 4S with regards to the MDI. Bluetooth connects in exactly the same way and phonebook sync works.
Tried out the SIRI voice command stuff today in the car, works pretty well. Strange that the RNS510 shows "voicemail" when you are using SIRI. Would be great if I could initiate SIRI by using the steering wheel button but that is pushing it a bit I think!