With the release of iOS 6 Beta 3 in July, Apple already started rolling out its new @iCloud email addresses starting with users creating new Apple IDs or enabling mail on iCloud for the first time. Since then, Apple has started rolling out @iCloud addresses to existing @me.com and @mac.com users, reserving users the same address but with the iCloud.com domain. Today, Apple is sending out notification emails to more iCloud users to inform them an iCloud email address has been reserved and is ready to access.
Apple provided instructions for switching to the address here.
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