The iPhone 4S is absolutely compatible with the SoundDock II, as well as with any of our other current SoundDock models. This means it will be able to both charge and play. Additionally, our current SoundDock systems are also iPhone-certified, so they have been officially tested to work with the iPhone. Thanks for the question!
(This answer is true for the Bose SoundDock Portable and SoundDock 10 as well.)