Public launch September 17 ahead of iPhone 6 debut.
3G iPhones and the FCC Myth
The problem with this theory is that Apple frequently announces new products that don't first show up in the FCC database. Also, companies are allowed to have some degree of confidentiality for products.
Apple submitted the original iPhone to the FCC on March 9, 2007. The documents, however, remained confidential until May 17, 2007, about six weeks prior to its launch. So while it may have required months for approval, it only appeared in the FCC database for all to see about 6 weeks prior to the ship date.
In another example, Apple's Time Capsule was submitted to the FCC on January 15, 2008 -- the same day as its announcement. The documents were immediately public and the product again shipped about 6 weeks later.
If this rough pattern continues, we could see the 3G iPhone shipping by late July if Apple announces it (for example) at WWDC on June 10th.