The iPhone 4S as you see it was originally planned to be released at WWDC with iOS 5, like every iPhone before it [...] But something happened around February of this year that threw everything off. Apple was still integrating the Siri team and code into iOS and it was going much slower than planned. At the same time, Apple’s iPhone 5 (teardrop) plans were moving along on or ahead of schedule and the first prototypes were testing well. With mid-October being the earliest possible date of a Siri-fied iOS being ready – with “Apple-levels” of polish –Apple had to look at its options.
This could explain the iPhone 5 case leak-a-palooza) we saw just before the iPhone 4S announcement.
TiPb's own sources are split on this, with some saying Apple had planned iPhone 5 as the new iPhone, and iPhone 4S as souped up iPhone 4 for the mid-range market. Others insist iPhone 4S was the plan all along, and no redesign was ever in the cards for this year (or at least, wasn't considered for long).
9to5Mac's sources expect the redesigned iPhone to be announced in summer of 2012. TiPb's still hearing fall 2012, but this far out Apple can and will keep plans as flexible as possible.
Hit the link below for a full breakdown.