Preview at WWDC likely in June, followed by September launch.
More Evidence of iPhone OS 4.0 Already in Testing
One iPhone developer has since noted a single instance of iPhone OS 4.0 being used in their Pinch Media analytics.
The new operating system first appeared this week in their application's logs. While such data can still be faked, it seems less likely to be. Testing of iPhone OS 4.0 may correspond with reports of new iPhone hardware also being tested. Any new iPhone would require a new version of the iPhone operating system to support the new hardware. Apple is widely expected to release a new iPhone in the summer around WWDC 2010.