The answer is plain and simple, yes. All current iPhone 4 cases, sleeves, speaker docks and other accessories of the sorts will indeed fit and be fully compatible with the latest iPhone 4S. Be sure to check out our Gadget Hub for some iPhone 4 and 4S case reviews and guides!
Update: Correction, all iPhone 4 cases that are Verizon and AT&T compatible will fit the iPhone 4S as well. Both the Verizon iPhone 4 and iPhone 4S have a silent switch that is designed just a few millimeters towards the top and that has caused cases to have a slight longer cutout for the side volume buttons and silent switch. Most if not all iPhone 4 cases are now compatible with the iPhone 4S.