It looks like early skepticism about the iPhone 4S' evolutionary improvements set hasn't dimmed consumers' enthusiasm for the device. As of this morning, all three U.S. carriers were sold out of the $199, 16GB-capacity of the device, just a day before it's scheduled to ship.
Both Verizon and Sprint stopped taking preorders for the 16GB iPhone 4S, and AT&T is reporting a 21- to 28-day wait for preordered units. That said, you should still be able to purchase an iPhone 4S at retail tomorrow. Sprint's site even says "Sold Out. Available in Stores October 14th" next to the iPhone 4S 16GB listing.
If you're willing to spend an extra $100 or $200, you can still preorder the 32GB or 64GB versions of the iPhone 4S at Sprint.com. Verizon was sold out online of all iPhone 4S capacities. AT&T reports the same 21 to 28 day wait time for all models.