Prices are $649 for the 16GB iPhone, $749 for 32GB, and $849 for 64GB. All are shipping in one to two weeks. The unlocked phones only work for GSM carriers, so Verizon and Sprint customers should stay away, even though the iPhone 4S hardware supports CDMA.
You should also note that as of today, Sprint's iPhone 4S is SIM-locking all of its iPhones. They had originally shipped as SIM-unlocked, but now the lock occurs "invisibly" at activation, so you're locked in, just like the other carriers' phones. When in Cupertino, I guess. [Apple, AppleInsider]