So -- now, connect the dots:
1.) Sprint comes to Apple and asks them to make a 4G WiMAX iPhone 5 for their network. They negotiate, Apple agrees but only after Sprint agrees to pay for the entire operation.
2.) Apple announces iOS5 with a secret lead-feature -- Assistant / Siri voice integration -- that requires the A5 chip.
3.) The LTE / 4G chipsets for Verizon & AT&T can't be made to work to Apple standards in time for the traditional iPhone release in Summer '11.
4.) Apple begins work on an updated iPhone 4S, with A5 chipset, to allow a full-featured release of iOS5 across all carriers.
5.) The WiMAX iP5 is ready for release ahead of schedule from the LTE iP5
6.) Apple announces WiMAX iP5 for Sprint and A-5 enabled iP4S for Verizon and AT&T in October of 2011 -- allowing them to show off their lead feature (Siri) and release iOS5 to all carriers simultaneously.
7.) Sprint gets "exclusive" iP5 for Holiday season 2011
8.) Sometime in 2012, iP5 is released with LTE chips for AT&T and Verizon.