Preview at WWDC likely in June, followed by September launch.
First Online Retina MacBook Pro Orders Arriving
At least one of the readers who received his new MacBook Pro had ordered a custom configuration, bumping the high-end stock model to the 2.7 GHz processor and 16 GB of RAM.
Portion of label from delivered Retina MacBook Pro showing custom configuration
Newly-delivered Retina MacBook Pro on initial boot
A few customers have reported being able to obtain the new Retina MacBook Pro in-store, but many Apple retail stores still do not have any stock of the machines beyond the display models, with some now indicating that they will not have stock until Monday.
(Thanks, Trevor and Jeremy!)