Preview at WWDC likely in June, followed by September launch.
Apple Begins Shipping iPhone 4 to Customers
Several readers have notified us that they have started receiving shipment notifications and FedEx tracking numbers for their iPhone 4. Several of the shipment notifications indicate a "Delivers By" date of June 23rd -- a day before the official launch day.
We wouldn't read too much into these delivery dates as Apple has been known to artificially prolong ship times in order to match the worldwide launch date. The iPhone 4 is officially to be available on June 24th, 2010.