An iPhone 4S will be announced in a little bit, for sure. It was mentioned today on Apple's Ginza, Japan retail store page for release on October 14th. Two images titles "iPhone 4S" have appeared on the servers, and they show what is, essentially, an Verizon iPhone 4 running iOS5 with a more visible ambient light sensor on the black model.
Update: Apple’s Sapporo, Japan retail store page is now showing the same text as well. Run it through Google Translate to see what we mean.