Preview at WWDC likely in June, followed by September launch.
Retina MacBook Pro Shipping Estimates Improving for the First Time
Shipping estimates in Apple's other main distribution regions of the Americas and Europe/Middle East/Africa remain at 3-4 weeks, but it is likely they will see similar improvements in the near future.
While Apple announced the Retina MacBook Pro as immediately available on its debut day of June 11, shipping estimates quickly slipped to 2-3 weeks by the end of the day and reached 3-4 weeks roughly 24 hours later. Estimates have remained steady at 3-4 weeks since that time until today's improvement in the Asia-Pacific channel.
Update: The improved 2-3 week estimates have now spread to the Americas and are beginning to filter into a number of European stores.