On March 17th, 2009, Apple unveiled details about iPhone OS 3.0 software and released the new iPhone Software Development Kit for developers. The actual release was June 17th 2009, 3 months after the first preview version.
On April 8th, 2010, Apple unveils details about iPhone OS 4.0 software and releases the new iPhone Software Development Kit for developers. The actual release will be July 8th 2010, 3 months after the preview version ....(the last date is just a guesstimate)