It isn't about predicting product cycles
As seen in the Buyer's Guide, these can vary widely at times from product to product
It is about marketing strategy
My feeling is, all the speculation about a fall release for a 3rd generation iPad is more wishful thinking by those who are underwhelmed by the rumors of the 2nd generation's updates and feature set, and hopeful for yet another upgrade sooner rather than later
Kind of like how Verizon users kept on coming up with random dates for a Verizon iPhone? Their wishful thinking took 'em long enough but at least they finally got it.