Preview at WWDC likely in June, followed by September launch.
AT&T Unveiling New Blocking Service for Stolen Phones Next Week
AT&T issued this statement to MacRumors:
As announced in April, AT&T is creating a stolen phone database to prevent devices reported stolen from accessing wireless networks. We will install this availability next week for AT&T phones on our network and are working toward a cross-carrier solution later this year.The Verge notes that AT&T's guidance for staffers says "only the person who originated a block may request block removal". It's unclear how exactly the database will work, including preventing legitimate transfers in ownership from being flagged by malicious actors, but we expect to have more details from AT&T as the launch date for the stolen phone database comes closer.