It would be disappointing to see only a "4S" introduced instead of an iPhone "5".
It's been over a year since the iPhone 4 came out, which is a heck of a long time between releases. A slightly faster version with no other differences would be incredibly underwhelming.
Nothing wrong with the iPhone 4, but it's always nice to see that things are moving forward. Also while Apple offer an attractive package, other manufacturers are stepping up their game, and to sit back with a lazy (yes, lazy) update would be a little naive.
I sincerely hope that it's just a case of Apple keeping the iPhone 5 under heavy security. I don't really want to move away from Apple's sphere when I go shopping for a new smartphone.
edit: Rumors are constantly circulating that Apple are planning major redesigns for their hardware in 2012, maybe (if these rumors of the 4S are true), they are planning to present a whole new front all at once?