Preview at WWDC likely in June, followed by September launch.
Apple's Mac Pro Shipping Estimates Slip to April in Many Countries
This is the second time in two months that the Mac Pro has showed a change in shipping estimates, as Apple's stores around the world began to show a "March" shipping estimate last month. While the new shipping estimates do not necessarily reflect a worsening supply given that shipping delays have been persistent so far throughout the computer's launch, the changes do indicate that Apple has yet to make up significant ground as lead times appear to be remaining in the six-to-seven week range.
The new April shipping estimates are in effect for stock and custom configurations throughout Apple's online stores for Europe, Middle East, and Africa (EMEA) and Asia Pacific (APAC) operational regions. Estimates in Apple's stores for the Americas remain at March for the time being.
Update: Shipping estimates in the Americas have now also moved to April for all models.