While many of the iPhone 4 cases fit the 4S, case manufacturers are seeing the arrival of the 4S as a good opportunity to introduce new designs.
WaterField Designs, which is known for its high-quality sleeves for laptops and iPads, has just released a leather wallet that also does double-duty as an iPhone case. It's simply called the iPhone Wallet ($39) and it has a window (and sound port) that allows you to "see and hear your iPhone."
If you want something a little trimmer, WaterField's also serving up the iPhone Hint ($25), which the company describes as a "sleeve" that also has a window, as well as a pocket for credit cards.
Both the iPhone Wallet and iPhone Hint are available in brown or black.